#include <iostream>
#include <fstream>
#include <vector>
using namespace std;
ifstream in("dfs.in");
ofstream out("dfs.out");
const int N=100001;
bool viz[N];
int m,n;
vector <int> a[N];
void citire(){
    in>>n>>m;
    int i,x,y;
    for(i=1;i<m;i++)
    {
        in>>x>>y;
        a[x].push_back(y);
        a[y].push_back(x);
    }
    in.close();
}
void dfs(int x){
    viz[x]=true;
    int i,y;
    for(i=0;i<a[x].size();i++){
        y=a[x][i];
        if(!viz[y]){
            dfs(y);
        }
    }
}
int main()
{   citire();
    int k,cate=0;
    for(k=1;k<=n;k++){
        if(!viz[k]){
            dfs(k);
            cate++;
        }
    }
    out<<cate-1;
    return 0;
}