#include <bits/stdc++.h>

using namespace std;

const long long INF = 1LL << 60;

int main() {
    int N, M, P;
    long long Y;
    cin >> N >> M >> P >> Y;
    vector<vector<pair<int, int>>> G(N);
    for (int i = 0; i < M; i++) {
        int A, B, C;
        cin >> A >> B >> C;
        A--, B--;
        G[A].emplace_back(C, B);
        G[B].emplace_back(C, A);
    }
    vector<long long> dis(N, INF);
    using T = pair<long long, int>;
    priority_queue<T, vector<T>, greater<T>> q;
    dis[0] = 0;
    q.emplace(0, 0);
    while (!q.empty()) {
        long long d = q.top().first;
        int v = q.top().second;
        q.pop();
        if (dis[v] < d) continue;
        for (int i = 0; i < G[v].size(); i++) {
            long long nd = d + G[v][i].first;
            int nv = G[v][i].second;
            if (nd < dis[nv]) {
                dis[nv] = nd;
                q.emplace(nd, nv);
            }
        }
    }
    long long ans = 0;
    for (int i = 0; i < P; i++) {
        int D, E;
        cin >> D >> E;
        D--;
        ans = max(ans, (Y - dis[D]) / E);
    }
    cout << ans << endl;
    return 0;
}