#include <iostream>
#include <fstream>
#include <vector>

using namespace std;
ifstream f("dfs.in");
ofstream g("dfs.out");

int N, M;
vector<int> adj[100005];
int v[100005], cnt;

bool muchie(int x, int y){
    for(int i = 0; i < adj[x].size(); i++)
        if(adj[x][i] == y)
            return true;
    return false;
}

bool unsolved(){
    for(int i = 1; i <= N; i++)
        if(v[i] == 0)
            return true;
    return false;
}

void dfs(int i){
    v[i] = cnt;
    
    for(int j = 0; j < adj[i].size(); j++)
        if(v[adj[i][j]] == 0)
            dfs(adj[i][j]);
}

int main(){

    f >> N >> M;
    for(int i = 1; i <= M; i++){
        int x, y;
        f >> x >> y;
        adj[x].push_back(y);
        adj[y].push_back(x);
    }

    while(unsolved()){
        cnt++;
        int t = 0;
        while(v[t])
            t++;
        
        dfs(t);
    }

    g << cnt - 1;

    return 0;
}