from heapq import heappop, heappush


N, M, P, Y = map(int, input().split())
g = [[] for _ in range(N)]
for _ in range(M):
    u, v, c = map(int, input().split())
    u, v = u - 1, v - 1
    g[u].append((v, c))
    g[v].append((u, c))
dp = [0] * N
dp[0] = Y
hq = [(-Y, 0)]
while hq:
    d, u = heappop(hq)
    d = -d
    if dp[u] > d:
        continue
    for v, c in g[u]:
        if dp[v] < d - c:
            dp[v] = d - c
            heappush(hq, (-(d - c), v))
ans = 0
for _ in range(P):
    d, e = map(int, input().split())
    d -= 1
    ans = max(ans, dp[d] // e)
print(ans)