import java.util.*;

public class Main {
	
	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner(System.in);	
		int n = sc.nextInt();
		int m = sc.nextInt();
		int p = sc.nextInt();
		long y = sc.nextLong();
		Dijkstra djk = new Dijkstra(n);
		long[] t = new long[n + 1];
		for(int i = 1;i <= n;i++) {
			t[i] = Long.MAX_VALUE;
		}
		for(int i = 0;i < m;i++) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			int c = sc.nextInt();
			djk.addLine(a, b, c);
			djk.addLine(b, a, c);
		}for(int i = 0;i < p;i++) {
			int d = sc.nextInt();
			t[d] = sc.nextLong();
		}
		djk.solve(1,0);
		long ans = 0;
		for(int i = 1;i <= n;i++) {
			long comp = Math.max(0, y - djk.min[i])/t[i];
			ans = Math.max(ans, comp);
		}System.out.print(ans);
		
	}public static class Dijkstra {
		public PriorityQueue<Pair> q = new PriorityQueue<>();
		public long[] min;
		public boolean[] vis;
		public HashMap<Integer,HashSet<Pair>> nextNode = new HashMap<>(); 
		public int n;
		public  Dijkstra(int n) {
			this.n = n; 
			min = new long[n + 1];
			vis = new boolean[n + 1];
			for(int i = 1;i <= n;i++) {
				min[i] = Long.MAX_VALUE/2;
				nextNode.put(i, new HashSet<>());
			}
		}public void addLine(int from,int to,long cost) {
			nextNode.get(from).add(new Pair(cost,to));
		}
		public void solve(int ind,long val) {
			min[ind] = val;
			q.add(new Pair(val,ind));
			while(!q.isEmpty()) {
				Pair p = q.poll();
				if(vis[p.x])continue;
				vis[p.x] = true;
				for(Pair s:nextNode.get(p.x)) {
					if(min[s.x] > min[p.x] + s.v) {
						min[s.x] = min[p.x] + s.v;
						q.add(new Pair(min[s.x],s.x));
					}
				}
			}
		}public long getMin(int a) {
			return min[a];
		}
		public static class Pair implements Comparable<Pair> {
			long v;
			int x;
			public Pair(long a,int b) {
				v = a;
				x = b;
			}public int compareTo(Pair p) {
				if(this.v < p.v) {
					return -1;
				}if(this.v > p.v) {
					return 1;
				}return 0;
			}public boolean equals(Object o) {
				Pair p = (Pair)o;
				return p.x == this.x && p.v == this.v;
			}
		}
	}
}