import heapq

INF = 10**20

n, m, p, y = map(int, input().split())
edges = [[] for _ in range(n)]
for _ in range(m):
    a, b, c = map(int, input().split())
    a, b = a - 1, b - 1
    edges[a].append((c, b))
    edges[b].append((c, a))

stores = [INF] * n
for _ in range(p):
    d, e = map(int, input().split())
    stores[d - 1] = e

ans = 0
dist = [INF] * n
dist[0] = 0
que = [(0, 0)]
while que:
    cost, crt = heapq.heappop(que)
    if dist[crt] < cost:
        continue
    tmp = max(0, y - cost) // stores[crt]
    ans = max(ans, tmp)
    for nxt_cost, nxt in edges[crt]:
        if dist[nxt] <= cost + nxt_cost:
            continue
        dist[nxt] = cost + nxt_cost
        heapq.heappush(que, (dist[nxt], nxt))
print(ans)