#include<bits/stdc++.h>
using namespace std;

#include<atcoder/all>
using namespace atcoder;
using mint=atcoder::modint998244353;

#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#define int long long

template<class T>using pqmin=priority_queue<T,vector<T>,greater<T>>;

signed main(){
	int N,M,P,Y;cin>>N>>M>>P>>Y;
	vector<vector<pair<int,int>>> g(N);
	for(int i=0;i<M;i++){
		int a,b,c;cin>>a>>b>>c;
		a--;b--;
		g[a].push_back({b,c});
		g[b].push_back({a,c});
	}

	vector<int> v(N,1e18);
	for(int i=0;i<P;i++){
		int d,e;cin>>d>>e;
		v[d-1]=e;
	}

	vector<int> dist(N,1e18);

	dist[0]=0;
	pqmin<pair<int,int>> pq;
	pq.push({0,0});

	while(pq.size()){
		auto[cost,p]=pq.top();pq.pop();
		if(dist[p]<cost)continue;
		for(auto&&[e,c]:g[p]){
			if(dist[e]>dist[p]+c){
				dist[e]=dist[p]+c;
				pq.push({dist[e],e});
			}
		}
	}

	int ans=0;

	for(int i=0;i<N;i++){
		int tmp=max<int>(0,Y-dist[i]);
		ans=max(ans,tmp/v[i]);
	}

	cout<<ans<<endl;
}