#include <bits/stdc++.h>

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  int N, M, P; long long Y; std::cin >> N >> M >> P >> Y;
  std::vector<std::vector<std::pair<int, int>>> G(N);
  for (;M--;) {
    int a, b, c; std::cin >> a >> b >> c; a--, b--;
    G[a].emplace_back(b, c);
    G[b].emplace_back(a, c);
  }
  const long long INF = 1LL << 60;
  std::priority_queue<std::pair<long long, int>> pq;
  std::vector<long long> dist(N, INF);
  pq.emplace(0, 0);
  while (pq.size()) {
    auto [c, v] = pq.top(); pq.pop();
    c *= -1;
    if (dist[v] > c) {
      dist[v] = c;
      for (auto [v2, d] : G[v]) {
        pq.emplace(-(c + d), v2);
      }
    }
  }
  long long ans = 0;
  for (;P--;) {
    int D, E; std::cin >> D >> E; D--;
    ans = std::max(ans, (Y - dist[D]) / E);
  }
  std::cout << ans << '\n';
}