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

int N,M,P,Y;

signed main(){
  cin>>N>>M>>P>>Y;
  vector<vector<pair<int,int>>> G(N+1);
  for(int i = 0; i < M; i++){
    int a,b,c; cin>>a>>b>>c;
    G[a].push_back(make_pair(b,c));
    G[b].push_back(make_pair(a,c));
  }
  vector<int> hati(N+1,-100);
  for(int i = 0; i < P; i++){
    int d,e; cin>>d>>e;
    hati[d] = e;
  }
  priority_queue<pair<int,int>> Q;
  Q.push(make_pair(Y,1));
  vector<int> visited(N+1);
  vector<int> zan(N+1);
  while(!Q.empty()){
    int pos = Q.top().second;
    int cost = Q.top().first;
    Q.pop();
    if(visited[pos]) continue;
    visited[pos] = true;
    zan[pos] = cost;
    for(pair<int,int> i:G[pos]){
      int next = i.first;
      int katu = i.second;
      if(!visited[next]){
        if(cost - katu < 0) continue;
        Q.push(make_pair(cost - katu,next));
      }
    }
  }
  int ans = 0;
  for(int i = 1; i <= N; i++){
    ans = max(ans,zan[i]/hati[i]);
  }
  cout << ans << endl;
}