#include <bits/stdc++.h>
#include <bits/stdc++.h>
#include <bits/stdc++.h>
using namespace std;
// #include <atcoder/modint>
// using namespace atcoder;
// using mint = modint998244353;
using ll = long long;
#define fix(x) fixed << setprecision(x)
#define rep(i, n) for(int i = 0; i < n; ++i)
#define all(x) (x).begin(),(x).end()
template<class T>bool chmin(T&a, const T&b){if(a>b){a=b;return 1;}return 0;}
template<class T>bool chmax(T&a, const T&b){if(a<b){a=b;return 1;}return 0;}
constexpr ll INFLL = (1LL << 62), MOD = 998244353;
constexpr int INF = (1 << 30);
struct Edge{ int to,c; };
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n,m,p;
    ll y;
    cin >> n >> m >> p >> y;
    vector<vector<Edge>> g(n);
    rep(i,m){
        int a,b,c;
        cin >> a >> b >> c;
        --a; --b;
        g[a].emplace_back(b,c);
        g[b].emplace_back(a,c);
    }
    vector<ll> x(n,INFLL), dist(n,INFLL);
    rep(i,p){
        int d,e;
        cin >> d >> e;
        x[d-1] = e;
    }
    using P = pair<ll,int>;
    priority_queue<P,vector<P>,greater<P>> pq;
    pq.emplace(0,0);
    dist[0] = 0;
    while(pq.size()){
        auto [d,now] = pq.top();
        pq.pop();
        if(dist[now]!=d) continue;
        for(auto [to,c]:g[now]){
            if(chmin(dist[to],d+c)) pq.emplace(d+c,to);
        }
    }
    ll ans = 0;
    rep(i,n) chmax(ans, (y-dist[i])/x[i]);
    cout << ans << '\n';
    return 0;
}