#include <atcoder/lazysegtree.hpp>
using namespace atcoder;

#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i=0; i<n; i++)

#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
// #include <all.hpp>

#define int long long

signed main() {
  int n, m, p, y;
  cin >> n >> m >> p >> y;
  vector<vector<pair<int, int>>> g(n);
  rep(i, m) {
    int a, b, c;
    cin >> a >> b >> c;
    a--; b--;
    g[a].push_back({b, c});
    g[b].push_back({a, c});
  }
  vector<int> dist(n, 1e18);
  dist[0] = 0;
  priority_queue<pair<int, int>, vector<pair<int, int>>, greater<pair<int, int>>> pq;
  pq.push({0, 0});
  while(!pq.empty()) {
    auto [d, v] = pq.top(); pq.pop();
    if(dist[v] < d) continue;
    for(auto [nv, c] : g[v]) {
      if(dist[nv] > dist[v] + c) {
        dist[nv] = dist[v] + c;
        pq.push({dist[nv], nv});
      }
    }
  }
  int ans = 0;
  rep(i, p) {
    int d, e;
    cin >> d >> e;
    d--;
    chmax(ans, (y - dist[d]) / e);
  }
  cout <<ans << endl;
}