#include <iostream>
#include <fstream>
#include <vector>
#define nmax 100001
using namespace std;
vector<int>a[nmax];
int n,m,viz[nmax],v=1;

void citire()
{
    int i,x,y;
    scanf("%d%d",&n,&m);
    for(i=1; i<=n; i++)
      a[i].push_back(0);
    for(i=1; i<=m; i++)
    {
        scanf("%d%d",&x,&y);
        a[x].push_back(y);
        a[y].push_back(x);
    }
}

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


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


    return 0;
}