#include <iostream>
#include <fstream>
#include <queue>
#include <string.h>
#include <vector>
#include <algorithm>
using namespace std;

const int nmax = 100002;

int n,a,b,vis[nmax];
char inVar[nmax];
vector <int> v[nmax];

int bfs(){
    queue<int> Q;
    memset(vis,-1,4*(n+1));

    int done = 1, ck = 0;
    while (done <= n) {
        Q.push(done);
        vis[done] = ++ck;
        while (!Q.empty()) {
            int nod = Q.front();
            Q.pop();
            for (unsigned int i = 0; i < v[nod].size(); i++)
                if (vis[v[nod][i]] == -1) {
                    vis[v[nod][i]] = ck ;
                    Q.push(v[nod][i]);
                }
        }
        for(;done <= n && vis[done]!=-1;done++);
    }
    return ck-1;
}

int main()
{
    int m;
    freopen("dfs.in","r",stdin);
	freopen("dfs.out","w",stdout);
    scanf("%d %d %d %d",&n, &m, &a, &b);
    int x,y;
    for(int i = 0; i < m; i++) {
        scanf("%d %d",&x, &y);
        v[x].push_back(y);
        v[y].push_back(x);
    }

    y = bfs();

    printf("%d",y);
    fclose(stdin);
    fclose(stdout);
    return 0;
}