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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n, m;
    ll p, 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(b, c);
        g[b].emplace_back(a, c);
    }
    priority_queue<pair<ll,int>, vector<pair<ll,int>>, greater<pair<ll,int>>> pq;
    vector<ll> dp(n, 1ll << 60);
    dp[0] = 0;
    pq.emplace(0, 0);
    while(!pq.empty()){
        auto [d, v] = pq.top();
        pq.pop();
        if(d > dp[v]) continue;
        for(auto [u, w] : g[v]){
            if(d + w >= dp[u]) continue;
            dp[u] = d + w;
            pq.emplace(d + w, u);
        }
    }
    ll ans = 0;
    while(p--){
        int v, e;
        cin >> v >> e;
        v--;
        ans = max(ans, (y - dp[v]) / e);
    }
    cout << ans << '\n';
}