#include <iostream>
#include <iomanip>//小数点出力用
//cout << fixed << setprecision(10) << ans;
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <set>
#include <map>
using ll = long long;
using namespace std;
#define modP 998244353
bool chkrng0idx(int pos, int sup) { return (0 <= pos && pos < sup); }
int clk4(int num) { return (num - 2) * (num % 2); }
void yn(bool tf) { cout << (tf ? "Yes\n" : "No\n"); }

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int N, M;cin >> N >> M;
    int P;cin >> P;
    ll Y;cin >> Y;
    vector<pair<int, int>>nxt[200002];
    for (int i = 0;i < M;i++) {
        int A, B, C;cin >> A >> B >> C;
        nxt[A].push_back({ B,C });
        nxt[B].push_back({ A,C });
    }
    ll cost[200002];
    for (int i = 2;i <= N;i++)cost[i] = 2e18;
    bool ok[200002] = { 0 };
    priority_queue<pair<ll, int>>Q;
    Q.push({ 0,1 });
    while (!Q.empty()) {
        int now = Q.top().second;
        if (ok[now]) {
            Q.pop();
            continue;
        }
        ok[now] = 1;
        cost[now] = Q.top().first * (-1);
        Q.pop();
        for (int j = 0;j < nxt[now].size();j++) {
            if (ok[nxt[now][j].first])continue;
            if (cost[nxt[now][j].first] > cost[now] + nxt[now][j].second) {
                cost[nxt[now][j].first] = cost[now] + nxt[now][j].second;
                Q.push({ -1 * cost[nxt[now][j].first] ,nxt[now][j].first });
            }
        }
    }
    map<int, ll>MP;
    for(int i=0;i<P;i++){
        int D, E;cin >> D >> E;
        MP[D] = E;
    }
    ll ans = 0;
    for (int i = 1;i <= N;i++) {
        if (MP.find(i) == MP.end())continue;
        ans = max(ans, (Y - cost[i]) / MP[i]);
    }
    cout << ans;
    return 0;
}