#include <cstdio>
#include <vector>
#define lim 100001
using namespace std;
FILE *inFile = fopen("dfs.in","r");
FILE *outFile = fopen("dfs.out","w");
int nrN,nrM;
int viz[lim];
vector<int> graf[lim];

void citire()
{
    fscanf(inFile,"%d %d",&nrN,&nrM);
    for(int i = 0 ; i < nrM; i++)
    {
        int x,y;
        fscanf(inFile,"%d %d",&x,&y);
        graf[x].push_back(y);
        graf[y].push_back(x);
    }
}

void dfs(int nod)
{
    viz[nod] = 1;
    for(int i = 0; i < graf[nod].size();i++)
    {
        if(viz [graf[nod][i]] == 0)
            dfs(graf[nod][i]);
    }

}
int nr =0;
int main()
{
    citire();
    for(int i = 1; i <= nrN;i++)
    {
        if(viz[i] == 0)
        {
            dfs(1);
            nr++;
        }
    }

    fprintf(outFile,"%d\n",nr -1);
    return 0;
}