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

#ifdef LOCAL
#include <debug.hpp>
#else
#define debug(...)
#endif

// 到達不可: -1
template <class T>
vector<T> dijkstra(const vector<vector<pair<int, T>>>& G, int start = 0) {
    int N = G.size();
    vector<T> dist(N, T(-1));
    priority_queue<pair<T, int>, vector<pair<T, int>>, greater<pair<T, int>>> que;
    dist[start] = 0;
    que.emplace(dist[start], start);
    while (!que.empty()) {
        auto [expected, from] = que.top();
        que.pop();
        // 最短距離でない場合, continue
        if (dist[from] < expected) continue;
        for (auto&& [to, cost] : G[from]) {
            if (dist[to] == T(-1) || dist[to] > dist[from] + cost) {
                dist[to] = dist[from] + cost;
                que.emplace(dist[to], to);
            }
        }
    }
    return dist;
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
    ll N, M, P, Y;
    cin >> N >> M >> P >> Y;
    vector<vector<pair<int, ll>>> G(N);
    for (int i = 0; i < M; i++) {
        int A, B;
        ll C;
        cin >> A >> B >> C;
        A--, B--;
        G[A].emplace_back(B, C);
        G[B].emplace_back(A, C);
    }
    vector<ll> D(P), E(P);
    for (int i = 0; i < P; i++) cin >> D[i] >> E[i], D[i]--;

    auto dist = dijkstra(G, 0);
    ll ans = 0;
    for (int i = 0; i < P; i++) {
        if (dist[D[i]] == -1) continue;
        ans = max(ans, max(0LL, (Y - dist[D[i]])) / E[i]);
    }

    cout << ans << "\n";
}