#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n) - 1; i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r) - 1;i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, m, p;
	cin >> n >> m >> p;
	long long y; cin >> y;
	vector to(n, vector<P>());
	rep(i, m) {
		int a, b, c; cin >> a >> b >> c;
		a--, b--;
		to[a].emplace_back(b, c);
		to[b].emplace_back(a, c);
	}
	vector<P>s(p);
	rep(i, p) {
		cin >> s[i].first >> s[i].second;
		s[i].first--;
	}
	// cost
	vector<long long> dp(n, LINF);
	dp[0] = 0;
	std::priority_queue<std::pair<long long, int>, std::vector<std::pair<long long, int>>, std::greater<>> pq;
	pq.push({ dp[0], 0 });
	while (!pq.empty()) {
		auto p = pq.top();
		pq.pop();
		auto cost = p.first;
		int v = p.second;
		if (dp[v] < cost) continue;
		for (const auto &e : to[v]) {
			if (dp[e.first] > dp[v] + e.second) {
				dp[e.first] = dp[v] + e.second;
				pq.push({ dp[e.first], e.first });
			}
		}
	}
	long long ans = 0;
	rep(i, p) {
		auto[idx, v] = s[i];
		long long yy = y - dp[idx];
		chmax(ans, yy / v);
	}
	cout << ans << endl;
	return 0;
}