#include <bits/stdc++.h>

#define NMAX 100001

using namespace std;

ifstream fin("dfs.in");
ofstream fout("dfs.out");

int n, m, s, i, x, y, viz[NMAX], comp = 1;

vector<int> L[NMAX];

void DFS(int nod) {
    viz[nod] = comp;
    for (int &cnt : L[nod])
        if (!viz[cnt])
            DFS(cnt);
}

int main() {
    fin >> n >> m;
    for (i = 1; i <= m; i++) {
        fin >> x >> y;
        L[x].push_back(y);
        L[y].push_back(x);
    }

    for (i = 1; i <= n; i++) {
        if (!viz[i]) {
            DFS(i);
            comp++;
        }
    }

    fout << comp - 1;
    
    return 0;
}