output_description: Print the correct choice.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      A
      B
attempt: N, u, v = map(int, input().split())
  AB = [list(map(int, input().split())) for _ in range(N - 1)]
  
  x = [[] for _ in range(N)]
  for ab in AB:
      a = ab[0]
      b = ab[1]
      x[a - 1].append(b - 1)
      x[b - 1].append(a - 1)
  
  flag1 = [False] * N
  flag2 = [False] * N
  length1 = [0] * N
  length2 = [0] * N
  go = [v - 1]
  while go != []:
      a = go.pop()
      if flag1[a]:
          continue
      else:
          flag1[a] = True
          for i in x[a]:
              if flag1[i]:
                  continue
              else:
                  length1[i] = length1[a] + 1
                  go.append(i)
  
  go = [u - 1]
  while go != []:
      a = go.pop()
      if flag2[a]:
          continue
      else:
          flag2[a] = True
          for i in x[a]:
              if flag2[i]:
                  continue
              else:
                  length2[i] = length2[a] + 1
                  go.append(i)
  
  print(max(length1))
  
problem_description: Statement
  
  Takahashi is solving quizzes. He has easily solved all but the last one.
  
  The last quiz has three choices: 1, 2, and 3.
  
  With his supernatural power, Takahashi has found out that the choices A and B
  are both wrong.
  
  Print the correct choice for this problem.
samples: [{"input": "3\n    1", "output": "2\n    \n\nWhen we know 3 and 1 are both wrong, the correct choice is 2.\n\n* * *"}, {"input": "1\n    2", "output": "3"}]