#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define all(x) (x).begin(), (x).end()
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rrep(i, n) for(int i = (int)(n - 1); i >= 0; i--)
template <typename T> bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template <typename T> bool chmin(T &a,const T &b){if(a>b){a=b;return true;}return false;}
using ll = long long;
#define int ll
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vb = vector<bool>;
using vvb = vector<vector<bool>>;
using vs = vector<string>;
using pii = pair<int, int>;
/*
using mint = modint;
using vm = vector<mint>;
using vvm = vector<vector<mint>>;
*/

signed main(){
  const int inf = 1ll << 60;
  int n, m, p, y; cin >> n >> m >> p >> y;
  vector g(n, vector<pii>(0));
  while(m--){
  	int a, b, c; cin >> a >> b >> c; a--; b--;
  	g.at(a).emplace_back(b, c);
  	g.at(b).emplace_back(a, c);
  }
  vi shop(n, -1);
  while(p--){
  	int place, price; cin >> place >> price; place--;
  	shop.at(place) = price;
  }
  int ans = 0;
  priority_queue<pii, vector<pii>, greater<>> dijk; dijk.emplace(0, 0);
  vi d(n, inf); d.at(0) = 0;
  while(!dijk.empty()){
  	auto [cost, from] = dijk.top(); dijk.pop();
  	if(cost > d.at(from)) continue;
  	for(auto &[to, c] : g.at(from)){
  	  if(cost + c >= y) continue;
  	  if(!chmin(d.at(to), cost + c)) continue;
  	  dijk.emplace(cost + c, to);
  	}
  }
  rep(i, n) if(shop.at(i) != -1) chmax(ans, (y - d.at(i)) / shop.at(i));
  cout << ans << endl;
}