Connect cities

n = input().split(" ")
m = int(n[1])
n = int(n[0])

roads = []
visited = [True]
adj = [[]]
representatives = []

for i in range(n):
    visited.append(False)
    adj.append([])

for i in range(m):
    r = input().split(" ")
    r[0] = int(r[0])
    r[1] = int(r[1])
    roads.append(r)
    adj[r[0]].append(r[1])
    adj[r[1]].append(r[0])

def dfs(s):
    if visited[s]:
        return
    visited[s] = True
    for c in adj[s]:
        dfs(c)

x = 0
for i, c in enumerate(visited):
    if c == False:
        representatives.append(i)
        x += 1
        dfs(i)
print(x-1)
for i in range(x-1):
    print(representatives[i], representatives[i+1])

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 34, in <module>
    dfs(i)...

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 34, in <module>
    dfs(i)...

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 34, in <module>
    dfs(i)...

Test 9

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 34, in <module>
    dfs(i)...

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 34, in <module>
    dfs(i)...

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED