from collections import defaultdict
from heapq import heappop, heappush

INF = 10**18


def dijkstra(start, graph):
    n = len(graph)

    dist = [INF] * n
    dist[start] = 0

    checked = [False] * n

    pq = [(0, start)]

    while len(pq) > 0:
        now_d, now_v = heappop(pq)

        if checked[now_v]:
            continue

        checked[now_v] = True

        for next_v, cost in graph[now_v]:
            if checked[next_v]:
                continue

            next_d = now_d + cost

            if next_d >= dist[next_v]:
                continue

            dist[next_v] = min(next_d, dist[next_v])
            heappush(pq, (next_d, next_v))

    return dist


N, M, P, Y = map(int, input().split())

graph = [[] for _ in range(N)]
for _ in range(M):
    A, B, C = map(int, input().split())
    graph[A - 1].append((B - 1, C))
    graph[B - 1].append((A - 1, C))

shops = defaultdict(lambda: INF)
for _ in range(P):
    D, E = map(int, input().split())
    shops[D - 1] = min(shops[D - 1], E)


dist = dijkstra(0, graph)

ans = 0

for i in range(N):
    if shops[i] < INF:
        ans = max(ans, (Y - dist[i]) // shops[i])

print(ans)