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

#define LMAX 100010

int N, M, k=1;
vector<int> V[LMAX];
int viz[LMAX];

void dfs(int nod) {
    viz[nod] = k;
    for (int i=0, e=V[nod].size(); i<e; ++i)
        if (!viz[V[nod][i]])
            dfs(V[nod][i]);
}

int main () {

    int x,y;

    freopen("dfs.in","rt",stdin);
    freopen("dfs.out","wt",stdout);

    scanf("%d %d", &N, &M);
    for (int i=0; i<M; ++i) {
        scanf("%d %d", &x, &y);
        V[x].push_back(y);
        V[y].push_back(x);
    }

    for (int i=1; i<=N; ++i)
        if (!viz[i]) {
            dfs(i);
            ++k;
        }
    printf("%d", k-1);

    return 0;
}