#include <iostream>
#include <fstream>
#include <vector>
#define nmax 100002
using namespace std;
vector<int>a[nmax];
int n,m,viz[nmax],v=1,nr=0;

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

void dfs(int x)
{
    int i;
    viz[x]=v;
    for(i=1; i<=a[x].size()-1; 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)
    {
        v++;
        for(i=1; i<=n; i++)
            if(!viz[i])
            {
                dfs(i);
                nr++;
                break;
            }

    }
    cout<<v-1<<'\n';



    return 0;
}