//dfs(TLE解法)
//入力と逆順に探索する
#include <iostream>
#include <algorithm>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
ll N, M, P, Y;
vector<ll> rest;

vector<vector<pair<ll, ll>>> G;
void dfs(ll pos){
  for(ll i = G[pos].size() - 1 ; i >= 0; i--){
    auto [to, cost] = G[pos][i];
    if(rest[pos] >= cost && rest[pos] - cost > rest[to]){
      rest[to] = rest[pos] - cost;
      dfs(to);
    }
  }
}

int main(){
  cin >> N >> M >> P >> Y;
  G.resize(N + 1);
  for(int i = 1; i <= M; i++){
    ll a, b, c;
    cin >> a >> b >> c;
    G[a].push_back({b, c});
    G[b].push_back({a, c});
  }
  rest = vector<ll>(N + 1, -1);
  rest[1] = Y;
  dfs(1);
  
  ll ans = 0;
  for(int i = 1; i <= P; i++){
    ll d, e;
    cin >> d >> e;
    ans = max(ans, rest[d] / e);
  }
  cout << ans << endl;
  return 0;
}