//嘘ダイクストラ
#include <iostream>
#include <algorithm>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
const ll INF = 2000000000000000000;
ll N, M, P, Y;

//dijkstra
struct edge{
  ll pre, to;
  ll cost;
  ll idx;
};
vector<vector<edge>> G;
vector<ll> dijkstra(int start = 0) {
  int N = G.size();
  vector<ll> dist(N, INF);
  using classPQ = tuple<ll, ll, ll>;
  priority_queue<classPQ, vector<classPQ>, greater<classPQ>> que;
  que.emplace(0, start, 0);
  dist[start] = 0;
  while(!que.empty()){
    auto [d, c, p] = que.top();
    que.pop();
    
    if(dist[c] != d) continue;
    for(auto& e : G[c]){
      //=を含むとTLEする
      if(d + e.cost <= dist[e.to]){
        dist[e.to] = d + e.cost;
        que.emplace(dist[e.to], e.to, e.idx);
      }
    }
  }
  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(edge{a, b, c, i});
    G[b].push_back(edge{b, a, c, i});
  }
  auto dist = dijkstra(1);
  
  ll ans = 0;
  for(int i = 1; i <= P; i++){
    ll d, e;
    cin >> d >> e;
    if(dist[d] <= Y) ans = max(ans, (Y - dist[d]) / e);
  }
  cout << ans << endl;
  return 0;
}