#include <bits/stdc++.h>

typedef long long ll;
typedef std::pair<long long, long long> P;
typedef std::priority_queue<P, std::vector<P>, std::greater<P>> PQ;

int main()
{
    long long n, m, p, y;
    std::cin >> n >> m >> p >> y;
    std::vector<std::vector<P>> path(n);
    std::vector<long long> h(n, -1);
    for (int i = 0; i < m; ++i) {
        ll a, b, c;
        std::cin >> a >> b >> c;
        --a, --b;
        path[a].push_back({b, c});
        path[b].push_back({a, c});
    }
    for (int i = 0; i < p; ++i) {
        ll d, e;
        std::cin >> d >> e;
        --d;
        h[d] = e;
    }
    std::vector<long long> dist(n, 1e17);
    dist[0] = 0;
    PQ pq;
    pq.push({0, 0});
    while (!pq.empty()) {
        ll udist = pq.top().first;
        ll u = pq.top().second;
        pq.pop();
        if (udist > dist[u]) continue;
        for (auto edge : path[u]) {
            ll v = edge.first;
            ll c = edge.second;
            if (dist[v] > udist + c) {
                dist[v] = udist + c;
                pq.push({udist + c, v});
            }
        }
    }
    ll ans = 0;
    for (int i = 0; i < n; ++i) {
        if (h[i] == -1) continue;
        if (dist[i] > 1e16) continue;
        ll r = y - dist[i];
        ans = std::max(ans, r / h[i]);
    }
    std::cout << ans << std::endl;
}