#include <fstream>
#include <vector>
#include <queue>

#define NMAX 100005

using namespace std;

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

void dfs(int s);

vector<int>g[NMAX];
queue<int>q;

int n,m,s,i,a,b,uz[NMAX],k;

int main()
{

    cin >> n >> m;
    for(i=1; i<=m; i++)
    {
        cin >> a >> b;
        g[a].push_back(b);
        g[b].push_back(a);
    }
    for(i=1; i<=n; i++)
        if(uz[i] == 0)
        {
            k++;
            dfs(i);
        }
    cout << k-1;

    return 0;
}

void dfs(int s)
{
    uz[s]=1;
    for(i=0; i<g[s].size(); i++)
    {
        if(uz[g[s][i]] == 0)
        {
            dfs(g[s][i]);
        }
    }
}