#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return true; } return false; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return true; } return false; }
ll inf_ll = 9223372036854775807;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
using mint = atcoder::modint998244353;
using mint1 = atcoder::modint1000000007;
using Pa = std::pair<ll, ll>;

int Yes(bool x){
    if(x) cout << "Yes";
    else cout << "No";
    cout << endl;
    return 0;
}

int main(){
    ll N, M, P, Y;
    cin >> N >> M >> P >> Y;
    vector<vector<Pa>> G(N);
    rep(i, M){
        ll a, b, c;
        cin >> a >> b >> c;
        a--;
        b--;
        G[a].push_back({c, b});
        G[b].push_back({c, a});
    }
    vector<ll> A(N, inf_ll);
    rep(i, P){
        ll d, e;
        cin >> d >> e;
        d--;
        A[d] = e;
    }
    vector<ll> B(N, inf_ll);
    priority_queue<Pa, vector<Pa>, greater<Pa>> Q;
    B[0] = 0;
    Q.push({0, 0});
    while(!Q.empty()){
        auto [x, y] = Q.top();
        Q.pop();
        for(auto [i, j] : G[y]){
            if(B[j] > x+i){
                B[j] = x+i;
                Q.push({x+i, j});
            }
        }
    }
    ll ans = 0;
    rep(i, N){
        chmax(ans, (Y - B[i]) / A[i]);
    }
    cout << ans;
}