output_description: Print the answer.
  
  * * *
input_description: Input is given from Standard Input in the following format:
  
      
      
      N M
      A_1 B_1
      :
      A_M B_M
attempt: def f(x):
    while p[x]>0: x = p[x]
    return x
  N,M = map(int,input().split())
  p = [-1]*N
  for _ in range(M):
    A,B = map(lambda x:f(int(x)-1),input().split())
    if A==B: continue
    elif A<B: A,B=B,A
    p[A] += p[B]
    p[B] = A
  print(sum(i<0 for i in p)-1)
problem_description: Statement
  
  There are N cities numbered 1 through N, and M bidirectional roads numbered 1
  through M. Road i connects City A_i and City B_i.
  
  Snuke can perform the following operation zero or more times:
  
    * Choose two distinct cities that are not directly connected by a road, and build a new road between the two cities.
  
  After he finishes the operations, it must be possible to travel from any city
  to any other cities by following roads (possibly multiple times).
  
  What is the minimum number of roads he must build to achieve the goal?
samples: [{"input": "3 1\n    1 2", "output": "1\n    \n\nInitially, there are three cities, and there is a road between City 1 and City\n2.\n\nSnuke can achieve the goal by building one new road, for example, between City\n1 and City 3. After that,\n\n  * We can travel between 1 and 2 directly.\n  * We can travel between 1 and 3 directly.\n  * We can travel between 2 and 3 by following both roads (2 \\- 1 \\- 3)."}]