#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

ifstream r("dfs.in");
ofstream w("dfs.out");

vector<int> v[200001];
vector<int> viz;
int i, n, m, x, y, c = 0;

void dfs(int nod){
    viz[nod] = 1;
    for(i=0;i<v[nod].size();i++)
        if(viz[v[nod][i]]==0)
            dfs(v[nod][i]);
}
int main()
{
    r>>n>>m;
    for(i=0; i<m; i++){
        r>>x>>y;
        v[x].push_back(y);
        v[y].push_back(x);
    }
    viz.resize(n+1, 0);
    for(i=1; i<=n; i++)
        if(viz[i] == 0){
            c++; dfs(i);
        }
    w<<c-1;
    return 0;
}