#include <bits/stdc++.h>
#include <atcoder/all>
using mint = atcoder::static_modint<998244353>;
// using mint = atcoder::static_modint<1000000007>;
using namespace std;
using namespace atcoder;
using ld = long double;
using ll = long long;
#define mp(a,b) make_pair(a,b)
#define rep(i,s,n) for(int i=s; i<n; i++)
const vector<int> dx{1,0,-1,0},dy{0,1,0,-1};
struct dijkstra{
    public:
    dijkstra():_n(0){}
    dijkstra(int n):_n(n),G(n){}
    void add_edge(int from,int to,ll d){
        G[from].push_back({to,d});
    }
    vector<ll> start_from(int start){
        vector<ll> dist(_n,-1);
        priority_queue<pair<ll,int>,vector<pair<ll,int>>,greater<pair<ll,int>>> Q;
        Q.push({0,start});
        dist[start]=0;
        while(!Q.empty()){
            auto p=Q.top();
            Q.pop();
            int now=p.second;
            ll d=p.first;
            if(d!=dist[now])continue;
            for(auto e:G[now]){
                int to=e.first;
                ll w=e.second;
                if(dist[to]==-1 || w+d<dist[to])Q.push(mp(w+d,to)),dist[to]=w+d;
            }
        }
        return dist;
    }
    private:
        int _n;
        vector<vector<pair<int,ll>>> G;
};
int main(){
    ll n,m,p,y;cin >> n >> m >> p >> y;
    dijkstra G(n);
    rep(i,0,m){
        int a,b;cin >> a >> b;
        ll c;cin >> c;
        a--,b--;
        G.add_edge(a,b,c);
        swap(a,b);
        G.add_edge(a,b,c);
    }
    auto dist=G.start_from(0);
    ll ans=0;
    rep(i,0,p){
        ll d,e;cin >> d >> e;
        d--;
        ans=max(ans,max(0LL,y-dist[d])/e);
    }
    cout << ans;
}