#include <fstream>
#include <vector>

using namespace std;

ifstream cin("dfs.in");
ofstream cout("dfs.out");

vector <vector <int> > a;

int n, m, p = 1, viz[100005];

void dfs(int x);

int main()
{
    cin >> n >> m;

    a.resize(n + 2);

    for(int i = 1; i <= m; i++)
    {
        int x, y;
        cin >> x >> y;

        a[x].push_back(y);
        a[y].push_back(x);
    }

    for(int i = 1; i <= n; i++)
        if(viz[i] == 0)
        {
            dfs(i);
            p++;
        }

    cout << p - 1;

    return 0;
}

void dfs(int x)
{
    viz[x] = p;
    for(auto vecin : a[x])
        if(viz[vecin] == 0)
            dfs(vecin);
}