#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)(s); i < (int)(e); ++i)
#define all(a) (a).begin(),(a).end()

const ll INF = 1ll << 60;

template<typename T> bool chmin(T& a, const T& b) {
    if (a > b) {
        a = b;
        return true;
    }
    else return false;
}

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    
    int N, M, P;
    ll Y;
    cin >> N >> M >> P >> Y;
    
    vector<vector<pair<int, ll>>> G(N);
    rep(i, 0, M) {
        int A, B;
        ll C;
        cin >> A >> B >> C;
        --A, --B;
        G[A].push_back({B, C});
        G[B].push_back({A, C});
    }
    
    vector<ll> dist(N, INF);
    priority_queue<pair<ll, int>, vector<pair<ll, int>>, greater<pair<ll, int>>> que;
    dist[0] = 0;
    que.push({0, 0});
    
    while (!que.empty()) {
        ll d = que.top().first;
        int v = que.top().second;
        que.pop();
        
        if (d > dist[v]) continue;
        
        for (auto e : G[v]) {
            if (chmin(dist[e.first], dist[v] + e.second)) {
                que.push({dist[e.first], e.first});
            }
        }
    }
    
    ll ans = 0;
    
    rep(i, 0, P) {
        int D;
        ll E;
        cin >> D >> E;
        --D;
        
        ans = max(ans, (Y - dist[D]) / E);
    }
    
    cout << ans << '\n';
}