#include <fstream>
#include <iostream>
using namespace std;
ifstream f("dfs.in");
ofstream g("dfs.out");
int x,n,k=0,y;
long m,i;
int v[1001];
int a[1001][1001];
void dfs(int x){
    v[x]=1;
    for(i=1;i<=n;i++)if (v[i]==0&&a[x][i]==1) dfs(i);
}
int main()
{
    f>>n>>m;
    for (i=1;i<=m;i++){
        f>>x>>y;
        a[x][y]=a[y][x]=1;
    }
    x=1;
    while(x<=n){
        x++;
        if(v[x]==0){
            dfs(x);
            k++;
        }
    }
    k--;
    g<<k;
    g.close();
}