#include <bits/stdc++.h>
#include <atcoder/all>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/multiprecision/cpp_int.hpp>
namespace mp = boost::multiprecision;
using Bint = mp::cpp_int;
using namespace std;
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

int main()
{
    ll n, m, p, y;
    cin >> n >> m >> p >> y;
    vector<vector<P>> 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<int> d(p), e(p);
    rep(i,p) cin >> d[i] >> e[i];
    rep(i,p)--d[i];
    vector<ll> dist(n, 1e18);
    dist[0] = 0;
    priority_queue<P, vector<P>, greater<P>> pq;
    pq.emplace(dist[0], 0);
    while(!pq.empty()) {
        auto [D, i] = pq.top();pq.pop();
        if(D!=dist[i]) continue;
        for(auto [v, cost] : g[i]) {
            if(dist[v]>dist[i]+cost) {
                dist[v] = dist[i] + cost;
                pq.emplace(dist[v], v);
            }
        }
    }
    ll ans = 0;
    rep(i,p) {
        ll ret = max(0ll, y-dist[d[i]]);
        ans = max(ans, ret/e[i]);
    } 
    cout << ans << endl;
    return 0;
}