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

using ll = long long;
#define int ll

#undef INT_MAX
#define INT_MAX LLONG_MAX

using xy = array<int, 2>;

signed main() {
  int n, m, p, y; cin >> n >> m >> p >> y;
  vector<int> r(n, INT_MAX);
  vector<vector<xy>> v(n, vector<xy>(0));
  for (int i = 0; i < m; i++) {
    int a, b, c; cin >> a >> b >> c; --a; --b;
    v[a].push_back({b, c});
    v[b].push_back({a, c});
  }
  
  for (int i = 0; i < p; i++) {
    int a, b; cin >> a >> b; --a;
    r[a] = min(r[a], b);
  }
  
  priority_queue<xy, vector<xy>, greater<xy>> q;
  q.push({0, 0});
  vector<int> dist(n, INT_MAX);
  while (!q.empty()) {
    auto [c, f] = q.top(); q.pop();
    dist[f] = c;
    for (auto [k, d] : v[f]) {
      if (dist[k] == INT_MAX) {
        q.push({c+d, k});
      }
    }
    while (!q.empty() && dist[q.top()[1]] != INT_MAX) q.pop();
  }
  
  int ans = 0;
  
  for (int i = 0; i < n; i++) {
    if (r[i] == INT_MAX) continue;
    ans = max(ans, (y - dist[i]) / r[i]);
  }
  cout << ans << endl;
}