Establish equality

from collections import deque
n, m = map(int, input().split())
g = [[] for _ in range(n * m)]
used = [False for _ in range(n * m)]

start, finish = -1, -1

s_prev = ''
s = ''

for i in range(n):
    s_prev = s
    s = input()
    for j in range(m):
        if s[j] == "#":
            continue
        if i != 0 and s_prev[j] != '#':
            g[i * m + j].append((i - 1) * m + j)
            g[(i - 1) * m + j].append(i * m + j)
        if j != 0 and s[j - 1] != '#':
            g[i * m + j].append(i * m + j - 1)
        if j != m - 1 and s[j + 1] != '#':
            g[i * m + j].append(i * m + j + 1)

        if s[j] == 'A':
            start = i * m + j
        elif s[j] == 'B':
            finish = i * m + j

dist = [-1] * (n * m)
parent = [-1] * (n * m)

def bfs(v):
    global g, dist
    queue = deque([v])
    while queue:
        node = queue.popleft()
        for w in g[node]:
            if dist[w] == -1:
                parent[w] = node
                dist[w] = dist[node] + 1
                if w == finish:
                    break
                queue.append(w)



dist[start] = 0
bfs(start)

if dist[finish] == -1:
    print("NO")
else:
    print("YES")
    print(dist[finish])
    path = []
    path.append(finish)
    v = finish
    while parent[v] != start:
        path.append(parent[v])
        v = parent[v]
    path.append(start)
    path.reverse()
    for i in range(1, len(path)):
        if path[i] == path[i - 1] + 1:
            print('R', end='')
        elif path[i] == path[i - 1] - 1:
            print('L', end='')
        elif path[i] < path[i - 1]:
            print('U', end='')
        else:
            print('D', end='')

Test 1

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 2

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 3

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 4

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 5

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 6

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 9

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 11

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 12

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 13

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 14

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 15

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 16

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 17

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 18

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 19

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 20

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 21

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 22

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 23

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 24

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 25

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 26

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 27

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 28

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 29

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 30

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 31

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 32

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 33

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 34

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 35

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 36

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 37

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 38

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 39

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 40

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 41

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 42

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 43

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 44

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 45

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 46

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 47

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 48

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 49

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 50

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 51

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 52

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 53

Verdict: RUNTIME ERROR

Traceback (most recent call last):
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    n, m =...

Test 54

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...

Test 55

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 2, in <module>
    n, m =...