import types

_atcoder_code = """
# Python port of AtCoder Library.

__version__ = '0.0.1'
"""

atcoder = types.ModuleType('atcoder')
exec(_atcoder_code, atcoder.__dict__)

_atcoder_dsu_code = """
import typing


class DSU:
    '''
    Implement (union by size) + (path compression)
    Reference:
    Zvi Galil and Giuseppe F. Italiano,
    Data structures and algorithms for disjoint set union problems
    '''

    def __init__(self, n: int = 0):
        self._n = n
        self.parent_or_size = [-1] * n

    def merge(self, a: int, b: int) -> int:
        assert 0 <= a < self._n
        assert 0 <= b < self._n

        x = self.leader(a)
        y = self.leader(b)

        if x == y:
            return x

        if -self.parent_or_size[x] < -self.parent_or_size[y]:
            x, y = y, x

        self.parent_or_size[x] += self.parent_or_size[y]
        self.parent_or_size[y] = x

        return x

    def same(self, a: int, b: int) -> bool:
        assert 0 <= a < self._n
        assert 0 <= b < self._n

        return self.leader(a) == self.leader(b)

    def leader(self, a: int) -> int:
        assert 0 <= a < self._n

        if self.parent_or_size[a] < 0:
            return a

        self.parent_or_size[a] = self.leader(self.parent_or_size[a])
        return self.parent_or_size[a]

    def size(self, a: int) -> int:
        assert 0 <= a < self._n

        return -self.parent_or_size[self.leader(a)]

    def groups(self) -> typing.List[typing.List[int]]:
        leader_buf = [self.leader(i) for i in range(self._n)]

        result = [[] for _ in range(self._n)]
        for i in range(self._n):
            result[leader_buf[i]].append(i)

        return list(filter(lambda r: r, result))
"""

atcoder.dsu = types.ModuleType('atcoder.dsu')
exec(_atcoder_dsu_code, atcoder.dsu.__dict__)
DSU = atcoder.dsu.DSU

# from atcoder.dsu import DSU
import sys

def main():
  pin=sys.stdin.readline
  N,M=map(int,pin().split())
  uf=DSU(N)
  for i in range(M):
    A,B=map(int,pin().split())
    uf.merge(A,B)
  ans=len(uf.groups())
  print(ans-1)
  return
main()

### FIXED CODE ###

'''
Created on 2020/09/26

@author: harurun
'''
#UnionFind(要素数)で初期化
class UnionFind():
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n
        #parents 各要素の親番号を格納するリストを返す

    #要素xが属するグループを返す
    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]
    #要素xとyが属するグループを併合する
    def union(self, x, y):
        x = self.find(x)
        y = self.find(y)

        if x == y:
            return

        if self.parents[x] > self.parents[y]:
            x, y = y, x

        self.parents[x] += self.parents[y]
        self.parents[y] = x

    #要素xが属するグループのサイズ
    def size(self, x):
        return -self.parents[self.find(x)]

    #要素xとyが同じグループに属するかどうか
    def same(self, x, y):
        return self.find(x) == self.find(y)

    #要素xが属するグループを返す
    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    #すべての根の要素
    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    #グループの数
    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        return {r: self.members(r) for r in self.roots()}

    def __str__(self):
        return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

def main():
  import sys
  pin=sys.stdin.readline
  pout=sys.stdout.write
  perr=sys.stderr.write

  N,M=map(int,pin().split())
  ans=0
  uf=UnionFind(N)
  for i in range(M):
    A,B=map(int,pin().split())
    uf.union(A-1, B-1)
  print(uf.group_count()-1)
  return 
main()