//bfs(TLE解法)
#include <iostream>
#include <algorithm>
#include <atcoder/all>
#include <cassert>
#include <set>
using namespace std;
using namespace atcoder;
using ll = long long;
ll N, M, P, Y;

vector<vector<pair<ll, ll>>> G;
vector<ll> bfs(ll start = 0) {
  ll n = G.size();
  vector<ll> dist(n, -1);
  queue<ll> que;
  que.emplace(start);
  dist[start] = Y;
  while(!que.empty()){
    int pos = que.front(); que.pop();
    int r;
    for(ll i = 0; i < G[pos].size() / 2; i++){
      auto [to, cost] = G[pos][i];
      if(dist[pos] >= cost && dist[pos] - cost > dist[to]){
        dist[to] = dist[pos] - cost ;
        que.push(to);
      }
      r = (int)G[pos].size() - 1 - i;
      if(i >= r) break;
      auto [to2, cost2] = G[pos][r];
      if(dist[pos] >= cost && dist[pos] - cost > dist[to2]){
        dist[to2] = dist[pos] - cost ;
        que.push(to2);
      }
    }
  }
  return dist;
}

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});
  }
  auto ret = bfs(1);
  ll ans = 0;
  for(int i = 1; i <= P; i++){
    ll d, e;
    cin >> d >> e;
    ans = max(ans, ret[d] / e);
  }
  cout << ans << endl;
  return 0;
}