#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef vector<VL> VVL;
typedef long long LL;
#define all(a) (a).begin(), (a).end()
#define Yes(n) cout << ((n) ? "Yes" : "No"  ) << endl
#define ALL(a)  (a).begin(),(a).end()
#define pb push_back

int main() {
  LL n,m,p,y;
  cin>>n>>m>>p>>y;
  VVI A[n];
  rep(i,m){
    int a,b,c; cin>>a>>b>>c;
    a--;b--;
    A[a].pb({b,c});
    A[b].pb({a,c});
  }
  LL dist[n];
  rep(i,n){dist[i]=-1;}
  set<VL> D;
  D.insert({-y,0});
  while(D.size()>0){
    VL z=*D.begin(); D.erase(z);
    if(dist[z[1]]!=-1){continue;}
    int i=z[1]; dist[i]=-z[0];
    for(VI zz : A[i]){
      int j=zz[0];
      if(dist[j]==-1){
        D.insert({-max(dist[i]-zz[1],LL(0)),j});
      }
    }
  }
  LL ans=0;
  rep(i,p){
    int d,e;cin>>d>>e;d--;
    ans=max(ans,dist[d]/e);
  }cout<<ans<<endl;
}