#include<iostream>
#include<cstdio>
#include<queue>
#include<vector>
#include<cstring> 
#define mysister
#define maxn 100002
using namespace std;
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;
}