from heapq import heapify, heappush, heappop
N, M, P, Y = map(int, input().split())
graph = [{} for _ in range(N)]
for _ in range(M):
    a, b, c = map(int, input().split())
    a -= 1
    b -= 1
    graph[a][b] = c
    graph[b][a] = c
headband = [None for _ in range(N)]
for _ in range(P):
    d, e = map(int, input().split())
    d -= 1
    headband[d] = e

cost = [float('inf') for _ in range(N)]
cost[0] = 0
Q = [(0, 0)]
heapify(Q)

maximum = 0
while Q:
    c, v = heappop(Q)
    if c > cost[v]:
        continue
    if headband[v]:
        maximum = max(maximum, max(Y - cost[v], 0) // headband[v])
    for u in graph[v]:
        d = c + graph[v][u]
        if d < cost[u]:
            cost[u] = d
            heappush(Q, (d, u))
print(maximum)