#include<iostream>
#include<cstdio>
#include<vector>
#include<queue>
#include<cstring>
#include<algorithm>
using namespace std;
struct cc
{
    int used,time;
}node[100005];
vector<int> G[100005];
queue<int> que;
int n,p,c,m,ans;
int main()
{
	freopen("dividecandy.in","r",stdin);
	freopen("dividecandy.out","w",stdout);
    scanf("%d%d%d%d",&n,&p,&c,&m);
    for(int i=1;i<=p;i++)
	{
        int x=0,y=0;
        scanf("%d%d",&x,&y);
        G[x].push_back(y);
        G[y].push_back(x);
    }
    for(int i=1;i<=n;i++) node[i].time=node[i].used=0;
    
    que.push(c);
    node[c].time=1;
    node[c].used=1;
    while(!que.empty())
	{
        int x=que.front();que.pop();
        
        for(int i=0;i<G[x].size()&&!G[x].empty();i++)
		{
            int k=G[x][i];
            if(node[k].used!=0) continue;
            node[k].time=node[x].time+1;
            node[k].used=1;
            ans=max(ans,node[k].time);
            que.push(k);
        }
    }
    printf("%d\n",ans+m);
    return 0;
}