#include <iostream>
#include <fstream>
#define nmax 5002
using namespace std;

int n,m,a[nmax][nmax],viz[nmax],v=1,nr=0;

void citire()
{
    int i,x,y;
    cin>>n>>m;
    for(i=1; i<=m; i++)
    {
        cin>>x>>y;
        a[x][++a[x][0]]=y;
        a[y][++a[y][0]]=x;
    }
}

void dfs(int x)
{
    int i;
    viz[x]=v;
    for(i=1; i<=a[x][0]; i++)
        if(!viz[a[x][i]])
        {
            dfs(a[x][i]);
            nr++;
        }
}


int main()
{
    freopen("dfs.in","rt",stdin);
    freopen("dfs.out","wt",stdout);
    int i;
    citire();
    while(nr<n)
    {
        for(i=1; i<=n; i++)
            if(!viz[i])
            {
                dfs(i);
                nr++;
                break;
            }
        v++;
    }
    cout<<v-1<<'\n';



    return 0;
}