from heapq import heappop, heappush

INF = 10**18

def dijkstra(start, graph):
    n = len(graph)
    dist = [INF] * n
    dist[start] = 0

    pq = [(0, start)]

    while pq:
        now_d, now_v = heappop(pq)

        if now_d > dist[now_v]:
            continue

        for next_v, cost in graph[now_v]:
            next_d = now_d + cost

            if next_d < dist[next_v]:
                dist[next_v] = next_d
                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 = [INF] * N
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 and dist[i] <= Y:
        ans = max(ans, (Y - dist[i]) // shops[i])

print(ans)