#include <stdio.h>
#include <vector>
using namespace std;

vector <int> g[100010];
int n,m,i,a,b,nri,v[100010];

void dfs(int nri)
{
	if(v[nri]==1){
		return;
	}
	v[nri]=1;
	for(i=0;i<g[nri].size();i++){
		dfs(g[nri][i]);
	}
}

int main()
{
	freopen("dfs.in","r",stdin);
	freopen("dfs.out","w",stdout);
	scanf("%d%d",&n,&m);
	for(i=1;i<=m;i++){
		scanf("%d%d",&a,&b);
		g[a].push_back(b);
		g[b].push_back(a);
	}
	for(i=1;i<=n;i++){
		if(v[i]==0){
			nri++;
			dfs(i);
		}
	}
	printf("%d",nri-1);
	return 0;
}