#include<bits/stdc++.h>
#include<iostream>
#include<cstdio>
#include<queue>
#include<vector>
#include<cstring> 
using namespace std;
const int maxn=100002;
vector<int>G[maxn];
queue<int>q;
bool inq[maxn];
int d[maxn];
int n,m,c,t,ans=-0x7fffffff;
int main(){
    freopen("dividecandy.in","r",stdin);
    freopen("dividecandy.out","w",stdout);
	scanf("%d%d%d\n%d",&n,&m,&c,&t);
	for(int i=1;i<=m;i++){
		int x,y;
		scanf("%d%d",&x,&y);
		G[x].push_back(y);
		G[y].push_back(x);
	}
	memset(d,0x7f,sizeof(d));
	d[c]=1;
	q.push(c);
	inq[c]=true;
	while(!q.empty()){
		int u=q.front();
		q.pop();
		inq[u]=false;
		int l=G[u].size();
		for(int i=0;i<l;i++){
			int v=G[u][i];
			if(d[u]<0x7fffffff&&d[v]>d[u]+1){
				d[v]=d[u]+1;
				ans=max(ans,d[v]);
				q.push(v);
				inq[v]=true;
			}
		}
	}
	ans+=t;
	printf("%d\n",ans);
	return 0;
}