#include <iostream>
#include <vector>
#include <queue>
#include <utility>
using namespace std;
using ll = long long;
using Graph = vector<vector<pair<int, ll>>>;
using pli = pair<ll, int>;

const ll INF = 1LL<<60;

Graph G(200010);
ll F[200010];

void Dijkstra(int src){
    fill(&F[0], &F[0]+200010, INF);
    F[src] = 0;
    priority_queue<pli, vector<pli>, greater<pli>> Q;
    Q.push({0, src});

    while(! Q.empty()){
        auto [cost, cur] = Q.top();
        Q.pop();
        if (cost>F[cur]) continue;
        for (auto [dst, money]: G[cur]){
            if (cost+money<F[dst]){
                Q.push({F[dst]=cost+money, dst});
            }
        }
    }
}

int main(){
    int N, M, P; ll Y; cin>>N>>M>>P>>Y;
    for (int i=0;i<M;i++){
        int A, B; ll C; cin>>A>>B>>C;
        G[A].push_back({B, C});
        G[B].push_back({A, C});
    }
    Dijkstra(1);

    ll ans = -1;
    for (int i=0;i<P;i++){
        int D; ll E; cin>>D>>E;
        if (F[D]==INF || F[D]>Y) continue;
        ans = max(ans, (Y-F[D])/E);
    }
    cout << ans << "\n";
}