Connect cities

n, m = [int(x) for x in input().split()]

graph = [[] for i in range(n + 1)]
for i in range(m):
    a, b = [int(x) for x in input().split()]
    graph[a].append(b)
    graph[b].append(a)

visited = [False] * (n + 1)
group = []

def dfs(city):
    visited[city] = True
    for i in graph[city]:
        if not visited[i]:
            dfs(i)

for i in range(1, n + 1):
    if not visited[i]:
        group.append(i)
        dfs(i)

print(len(group) - 1)
for i in range(1, len(group)):
    print(group[0], group[i])

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 21, in <module>
    dfs(i)...

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 21, in <module>
    dfs(i)...

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 21, in <module>
    dfs(i)...

Test 9

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 21, in <module>
    dfs(i)...

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 21, in <module>
    dfs(i)...

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED