using System;
using System.Collections.Generic;
using System.Linq;

class Edge
{
    public Edge(int f, int n, int c)
    {
        From = f;
        Next = n;
        Cost = c;
    }
    public int From = 0;
    public int Next = 0;
    public int Cost = 0;
}


class Program
{
    static void Main()
    {
        long N, M, P, Y;
        {
            long[] vs = Console.ReadLine().Split().Select(_ => long.Parse(_)).ToArray();
            N = vs[0];
            M = vs[1];
            P = vs[2];
            Y = vs[3];
        }

        List<Edge>[] lists = new List<Edge>[N];
        for (int i = 0; i < N; i++)
            lists[i] = new List<Edge>();

        for (int i = 0; i < M; i++)
        {
            int[] vs = Console.ReadLine().Split().Select(_ => int.Parse(_)).ToArray();
            int a = vs[0] - 1;
            int b = vs[1] - 1;
            int c = vs[2];
            lists[a].Add(new Edge(a, b, c));
            lists[b].Add(new Edge(b, a, c));
        }
        long[] costs = new long[N];
        for (int i = 0; i < N; i++)
            costs[i] = long.MaxValue;

        costs[0] = 0;
        Queue<int> q = new Queue<int>();
        q.Enqueue(0);
        while (q.Count > 0)
        {
            int cur = q.Dequeue();
            long cost = costs[cur];
            foreach (Edge edge in lists[cur])
            {
                long ncost = cost + edge.Cost;
                int next = edge.Next;
                if (costs[next] > ncost)
                {
                    costs[next] = ncost;
                    q.Enqueue(next);
                }
            }
        }
        long ans = 0;
        for (int i = 0; i < P; i++)
        {
            int[] vs = Console.ReadLine().Split().Select(_ => int.Parse(_)).ToArray();
            int a = vs[0] - 1;
            int c = vs[1];
            if (Y - costs[a] > 0)
                ans = Math.Max(ans, (Y - costs[a]) / c);
        }
        Console.WriteLine(ans);
    }
}