C++ :

#include <algorithm>
#include <cstdio>
using namespace std;

const int N = 1e5 + 5;

int n, m;
int f[N];
int ans;

int getf(int u) { return f[u] ? f[u] = getf(f[u]) : u; }

int main() {
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= m; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        if (getf(u) != getf(v)) f[getf(u)] = v;
    }
    for (int i = 1; i <= n; i++) ans += getf(i) == i;
    printf("%d\n", ans - 1);
    return 0;
}