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

struct Edge {
  int to, cost;
};

int main() {
  int N, M, P;
  long long Y, Ans = 0; cin >> N >> M >> P >> Y;
  vector<int> Shop(N, 2e9);
  vector<long long> seen(N, 1e18);
  vector<vector<Edge>> Graph(N);
  priority_queue<pair<long long, int>, vector<pair<long long, int>>, greater<pair<long long, int>>> PQ;

  for (int i = 0; i < M; i++) {
    int A, B, C; cin >> A >> B >> C;
    A--; B--;
    Graph.at(A).push_back({B, C});
    Graph.at(B).push_back({A, C});
  }
  for (int i = 0; i < P; i++) {
    int D, E; cin >> D >> E;
    D--;
    Shop.at(D) = E;
  }

  PQ.push({0, 0});
  while (!PQ.empty()) {
    pair<long long, int> nv = PQ.top();
    PQ.pop();
    if (seen.at(nv.second) != 1e18) continue;
    seen.at(nv.second) = nv.first;
    Ans = max(Ans, (Y - seen.at(nv.second)) / Shop.at(nv.second));
    for (Edge E : Graph.at(nv.second)) {
      if (seen.at(E.to) != 1e18) continue;
      PQ.push({seen.at(nv.second) + E.cost, E.to});
    }
  }

  cout << Ans << endl;
}