#include <bits/stdc++.h>
using namespace std;
int main()
{
    int N, M, P;
    long long Y;
    cin >> N >> M >> P >> Y;
    vector<int> A(M), B(M), C(M), D(P), E(P);
    vector<vector<pair<int, int>>> g(N + 1);
    for (int i = 0; i < M; ++i)
    {
        cin >> A[i] >> B[i] >> C[i];
        g[A[i]].push_back(make_pair(B[i], C[i]));
        g[B[i]].push_back(make_pair(A[i], C[i]));
    }
    for (int i = 0; i < P; ++i)
        cin >> D[i] >> E[i];
    priority_queue<pair<long long, int>, vector<pair<long long, int>>, greater<pair<long long, int>>> que;
    que.push(make_pair(0, 1));
    vector<long long> kakutei(N + 1, -1);
    while (que.size() > 0)
    {
        pair<long long, int> tmp = que.top();
        que.pop();
        if (kakutei[tmp.second] == -1)
        {
            kakutei[tmp.second] = tmp.first;
            for (auto x : g[tmp.second])
            {
                if (kakutei[x.first] == -1)
                    que.push(make_pair(tmp.first + x.second, x.first));
            }
        }
    }
    long long ans = 0;
    for (int i = 0; i < P; ++i)
    {
        long long tmp = max((long long)0, Y - kakutei[D[i]]);
        ans = max(ans, tmp / E[i]);
    }
    cout << ans << endl;
    return 0;
}