#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template<class T> using pqg = priority_queue<T, vector<T>, greater<T>>;

int main(){
    int N, M, P;
    ll Y;
    cin >> N >> M >> P >> Y;
    vector<vector<pair<int, int>>> G(N);
    for (int i = 0; i < M; i++){
        int u, v, w;
        cin >> u >> v >> w;
        u--, v--;
        G[u].emplace_back(v, w);
        G[v].emplace_back(u, w);
    }

    vector<int> D(P);
    vector<ll> E(P);
    for (int i = 0; i < P; i++) cin >> D[i] >> E[i], D[i]--;

    vector<ll> cost(N, LLONG_MAX);
    pqg<pair<ll, int>> pq;
    pq.emplace(0, 0);
    cost[0] = 0;
    while (!pq.empty()){
        auto [d, p] = pq.top();
        pq.pop();
        if (cost[p] < d) continue;
        for (auto [u, v] : G[p]){
            if (cost[u] > cost[p]+v){
                cost[u] = cost[p]+v;
                pq.emplace(cost[u], u);
            }
        }
    }

    ll ans = LLONG_MIN;
    for (int i = 0; i < P; i++){
        if (cost[D[i]] > Y) continue;
        ans = max(ans, (Y-cost[D[i]])/E[i]);
    }
    cout << ans << endl;;
}