from heapq import heappush, heappop

INF = 10 ** 18

def dijkstra(s, N): # (始点, ノード数)
    dist = [INF for _ in range(N)]
    hq = [(0, s)]
    dist[s] = 0
    seen = [False] * N # ノードが確定済みかどうか
    while hq:
        d, v = heappop(hq) # ノードを pop する
        if seen[v]:
            continue
        seen[v] = True
        for to, cost in G[v]: # ノード v に隣接しているノードに対して
            if dist[v] + cost < dist[to]:
                dist[to] = dist[v] + cost
                heappush(hq, (dist[to], to))
    return dist

import sys
input = sys.stdin.readline
N, M, P, Y = map(int, input().split())
G = [[] for _ in range(N)]
for _ in range(M):
    a, b, c = map(int, input().split())
    a-=1
    b-=1
    G[a].append((b, c))
    G[b].append((a, c))
D = dict()
for _ in range(P):
    d, e = map(int, input().split())
    d-=1
    D[d] = e

cost = dijkstra(0, N)
ans = 0
for i in range(N):
    if i not in D:
        continue
    if Y-cost[i]<=0:
        continue
    ans = max(ans, (Y-cost[i])//D[i])
print(ans)