#include<iostream>
using namespace std;
const int N=100010;
int p[N];
int find(int x)
{
	if(p[x]!=x) p[x]=find(p[x]);
	return p[x];
}
void merge(int x,int y)
{
	int xx=find(x);
	int yy=find(y);
	if(xx!=yy) p[xx]=yy;
}
int main(){
	int n,m;
	while(cin>>n>>m)
	{
		for(int i=1;i<=n;i++) p[i]=i;
		for(int i=1;i<=m;i++)
		{
			int x,y;
			cin>>x>>y;
			merge(x,y);
		}
		int sum=0;
		for(int i=1;i<=n;i++)
			if(p[i]==i)
				sum++;
		cout<<sum-1<<endl;
	}
	return 0;
}