#include <fstream>
#include <vector>

using namespace std;

ifstream fin("dfs.in");
ofstream fout("dfs.out");

const int NMAX = 100000;

int n, m, x, y, vecin, s, cc;
int viz[NMAX+5];
vector<int>muchii[NMAX+5];

void DFS(int nod, int cc)
{
    viz[nod]=cc;
    s=muchii[nod].size();
    for(int i=0; i<s; i++)
    {
        vecin=muchii[nod][i];
        if(!viz[vecin])
            DFS(vecin, cc);
    }
}

int main()
{
    fin>>n>>m;
    for(int i=1; i<=m; i++)
    {
        fin>>x>>y;
        muchii[x].push_back(y);
        muchii[y].push_back(x);
    }

    for(int i=1; i<=n; i++)
    {
        if(!viz[i])
        {
            cc++;
            DFS(i, cc);
        }
    }
    fout<<cc-1;
    return 0;
}