#include<iostream>
#include<vector>
#include<queue>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);++i)
using pl=pair<long,long>;
const long INF=3e18+100;
int main(){
	int N,M,P;
	long Y;
	cin>>N>>M>>P>>Y;
	vector<vector<pl>>G(N);
	rep(i,M){
		int a,b,c;
		cin>>a>>b>>c;
		a--;b--;
		G[a].push_back(make_pair(b,c));
		G[b].push_back(make_pair(a,c));
	}
	
	vector<long> d(N,INF);
	priority_queue<pl,vector<pl>,greater<pl>> pq;
	pq.push(make_pair(0,0));
	d[0]=0;
	while(!pq.empty()){
		auto [x,v]=pq.top();
		if(x>d[v]) continue;
		for(auto [to,cost]:G[v]){
			if(d[to]<=x+cost) continue;
			d[to]=x+cost;
			pq.push(make_pair(d[to],to));
		}
	}
	long ans=0;
	rep(i,P){
		long D,E;
		cin>>D>>E;
		D--;
		ans=max(ans, (Y-d[D])/E);
	}
	cout<<ans<<endl;
}