#!/usr/bin/env python3

from networkx.utils import UnionFind

import sys

def input(): return sys.stdin.readline().rstrip()





def main():

    n,m=list(map(int, input().split()))

    uf = UnionFind()

    for i in range(1,n+1):

        _=uf[i]

    for _ in range(m):

        a,b=list(map(int, input().split()))

        uf.union(a, b)  # aとbをマージ

    print((len(list(uf.to_sets()))-1))

    #for group in uf.to_sets():  # すべてのグループのリストを返す

        #print(group)



if __name__ == '__main__':

    main()



import sys

def input(): return sys.stdin.readline().rstrip()

from collections import defaultdict



class UnionFind():

    def __init__(self, n):

        self.n = n

        self.parents = [-1] * n



    def find(self, x):

        if self.parents[x] < 0:

            return x

        else:

            self.parents[x] = self.find(self.parents[x])

            return self.parents[x]



    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



    def size(self, x):

        return -self.parents[self.find(x)]



    def same(self, x, y):

        return self.find(x) == self.find(y)



    def members(self, x):

        root = self.find(x)

        return [i for i in range(self.n) if self.find(i) == root]



    def roots(self):

        return [i for i, x in enumerate(self.parents) if x < 0]



    def group_count(self):

        return len(self.roots())



    def all_group_members(self):

        self.all_group_member=defaultdict(list)

        for i in range(self.n):

            self.all_group_member[self.find(i)].append(i)

        return self.all_group_member



    def __str__(self):

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

 

def main():

    n,m=list(map(int, input().split()))

    uf = UnionFind(n+1)

    for _ in range(m):

        a,b=list(map(int, input().split()))

        uf.union(a, b)  # aとbをマージ

    print((uf.group_count()-2))

    #for group in uf.to_sets():  # すべてのグループのリストを返す

        #print(group)

 

if __name__ == '__main__':

    main()
#!/usr/bin/env python3
from networkx.utils import UnionFind
import sys


def input():
    return sys.stdin.readline().rstrip()


def main():
    n, m = list(map(int, input().split()))
    uf = UnionFind()
    for i in range(1, n + 1):
        _ = uf[i]
    for _ in range(m):
        a, b = list(map(int, input().split()))
        uf.union(a, b)  # aとbをマージ
    print((len(list(uf.to_sets())) - 1))
    # for group in uf.to_sets():  # すべてのグループのリストを返す
    # print(group)


if __name__ == "__main__":
    main()

import sys


def input():
    return sys.stdin.readline().rstrip()


from collections import defaultdict


class UnionFind:
    def __init__(self, n):
        self.n = n
        self.parents = [-1] * n

    def find(self, x):
        if self.parents[x] < 0:
            return x
        else:
            self.parents[x] = self.find(self.parents[x])
            return self.parents[x]

    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

    def size(self, x):
        return -self.parents[self.find(x)]

    def same(self, x, y):
        return self.find(x) == self.find(y)

    def members(self, x):
        root = self.find(x)
        return [i for i in range(self.n) if self.find(i) == root]

    def roots(self):
        return [i for i, x in enumerate(self.parents) if x < 0]

    def group_count(self):
        return len(self.roots())

    def all_group_members(self):
        self.all_group_member = defaultdict(list)
        for i in range(self.n):
            self.all_group_member[self.find(i)].append(i)
        return self.all_group_member

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


def main():
    n, m = list(map(int, input().split()))
    uf = UnionFind(n + 1)
    for _ in range(m):
        a, b = list(map(int, input().split()))
        uf.union(a, b)  # aとbをマージ
    print((uf.group_count() - 2))
    # for group in uf.to_sets():  # すべてのグループのリストを返す
    # print(group)


if __name__ == "__main__":
    main()

-#!/usr/bin/env python3 -from networkx.utils import UnionFind +from collections import defaultdict + + +class UnionFind: +    def __init__(self, n): +        self.n = n +        self.parents = [-1] * n + +    def find(self, x): +        if self.parents[x] < 0: +            return x +        else: +            self.parents[x] = self.find(self.parents[x]) +            return self.parents[x] + +    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 + +    def size(self, x): +        return -self.parents[self.find(x)] + +    def same(self, x, y): +        return self.find(x) == self.find(y) + +    def members(self, x): +        root = self.find(x) +        return [i for i in range(self.n) if self.find(i) == root] + +    def roots(self): +        return [i for i, x in enumerate(self.parents) if x < 0] + +    def group_count(self): +        return len(self.roots()) + +    def all_group_members(self): +        self.all_group_member = defaultdict(list) +        for i in range(self.n): +            self.all_group_member[self.find(i)].append(i) +        return self.all_group_member + +    def __str__(self): +        return "\n".join("{}: {}".format(r, self.members(r)) for r in self.roots()) + + -    uf = UnionFind() -    for i in range(1, n + 1): -        _ = uf[i] +    uf = UnionFind(n + 1) -    print((len(list(uf.to_sets())) - 1)) +    print((uf.group_count() - 2))