#include <fstream>
#include <vector>
using namespace std;
ifstream f("dfs.in");
ofstream g("dfs.out");

#define MAX 100001

vector <int> a[MAX];
int n, m, val, viz[MAX];

void read() {
    int x, y, i;
    f>>n>>m;
    for(i = 1; i <= m; i++) {
        f>>x>>y;
        a[x].push_back(y);
        a[y].push_back(x);
    }
}

void DFS(int x) {
    int i;
    viz[x] = val;
    for(i = 0; i < a[x].size(); i++) {
        if(!viz[a[x][i]])
            DFS(a[x][i]);
    }
}

int main()
{
    int i;
    read();
    val = 1;
    for(i = 1; i <= n; i++)
        if(!viz[i]) {
            DFS(i);
            val++;
        }
    g<<val - 1<<'\n';
    return 0;
}