#include<bits/stdc++.h>
using namespace std;
ifstream f("dfs.in");
ofstream g("dfs.out");

vector<int> v[100001];
//int vis[100001]={0};
vector<int> vis;
unsigned int n,m,i,nrcomp=0,x,y;

/*void add(int node1,int node2){
    v[node1].push_back(node2);
}*/
void dfs(int node){
    vis[node]=nrcomp;
    //cout<<node<<" ";
    for(i=0;i<v[node].size();++i){
        if(!vis[v[node][i]]){
            dfs(v[node][i]);
        }
    }
}
int main(){
    f>>n>>m;
    //cin>>n>>m;
    for(i=0;i<m;++i){
        //f>>x>>y;
        f>>x>>y;
        v[x-1].push_back(y-1);
        v[y-1].push_back(x-1);
    }
    for(i=0;i<n;i++)
    {
        vis.push_back(0);
    }
    //dfs(1);
    for(i=0;i<n;++i)
    {
        if(vis[i]==0){
            nrcomp++;
            dfs(i);
        }
    }
    g<<nrcomp-1;
    //cout<<nrcomp-1;
}