#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 < M; ++i){
		std::cin >> a >> b >> c; a--, b--;
		graph[a].emplace_back(c, b);
		graph[b].emplace_back(c, a);
	}
	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::priority_queue<std::pair<long long, int>> search;
	search.push({0, 0});
	while(!search.empty()){
		auto p = search.top();
		search.pop();
		if(p.first != distance[p.second]) continue;
		for(auto q : graph[p.second]){
			if(p.first - q.first > distance[q.second]){
				distance[q.second] = p.first - q.first;
				search.emplace(distance[q.second], q.second);
			}
		}
	}
	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;
}