#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using vi = vector<int>;
using vl = vector<ll>;
#define rep3(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define rep2(i, a, b) rep3(i, a, b, 1)
#define rep1(i, n) rep2(i, 0, n)
#define rep0(n) rep1(aaaaa, n)
#define ov4(a, b, c, d, name, ...) name
#define rep(...) ov4(__VA_ARGS__, rep3, rep2, rep1, rep0)(__VA_ARGS__)
#define per(i, a, b) for (ll i = (a) - 1; i >= (b); i--)
#define fore(e, v) for (auto&& e : v)
#define all(a) begin(a), end(a)
#define sz(a) (int)(size(a))
#define lb(v, x) (lower_bound(all(v), x) - begin(v))
#define eb emplace_back

template <typename T, typename S>
bool chmin(T& a, const S& b) {
  return a > b ? a = b, 1 : 0;
}
template <typename T, typename S>
bool chmax(T& a, const S& b) {
  return a < b ? a = b, 1 : 0;
}
const int INF = 1e9 + 100;
const ll INFL = 3e18 + 100;
#define i128 __int128_t
struct _ {
  _() { cin.tie(0)->sync_with_stdio(0), cout.tie(0); }
} __;

int main(){
  ll N,M,P,Y;
  cin>>N>>M>>P>>Y;
  using ai2=array<int,2>;
  vector G(N,vector(0,ai2()));
  rep(M){
    int A,B,C;
    cin>>A>>B>>C;
    --A,--B;
    G[A].eb(ai2{B,C});
    G[B].eb(ai2{A,C});
  }
  vl D(N,Y+1);
  rep(P){
    ll S,E;
    cin>>S>>E;
    D[S-1]=E;
  }
  vl dist(N,(ll)1e18);
  using al2=array<ll,2>;
  priority_queue<al2,vector<al2>,greater<al2>>pq;
  dist[0]=0;
  pq.push({0,0});
  while(pq.size()){
    auto [d,v]=pq.top();
    pq.pop();
    if(dist[v]<d)continue;
    for(auto [u,c]:G[v]){
      if(dist[u]>d+c){
        dist[u]=d+c;
        pq.push({d+c,u});
      }
    }
  }
  ll ans=0;
  rep(i,N){
    ans=max(ans,max(0ll,Y-dist[i])/D[i]);
  }
  cout<<ans<<'\n';
}