#include <iostream>
#include <vector>
#include <queue>

int main(){
	long long N, M, P, Y;
	std::cin >> N >> M >> P >> Y;
	std::vector<std::vector<std::pair<int, int>>> graph(N);
	int D[P], E[P];
	for(int i = 0, a, b, c; i < N; ++i){
		std::cin >> a >> b >> c; a--, b--;
		graph[a].emplace_back(b, c);
		graph[b].emplace_back(a, c);
	}
	for(int i = 0; i < P; ++i){
		std::cin >> D[i] >> E[i]; D[i]--;
	}
	std::vector<long long> distance(N, -0x8000000000000000); distance[0] = 0;
	std::vector<char> already(N, false);
	std::priority_queue<std::pair<long long, int>> search;
	search.push({0, 0});
	while(search.size() > 0){
		auto p = search.top();
		search.pop();
		if(already[p.second]) continue;
		already[p.second] = true;
		for(auto q : graph[p.second]){
			if(p.first - q.second > distance[q.first]){
				distance[q.first] = p.first - q.second;
				search.push({distance[q.first], q.first});
			}
		}
	}
	long long ans = 0;
	for(int i = 0; i < P; ++i){
		long long zankin = Y + distance[D[i]];
		if(zankin > 0){
			ans = std::max(ans, zankin / E[i]);
		}
	}
	std::cout << ans << std::endl;
	return 0;
}