n, m, p, y = map(int, input().split())
ABC = [tuple(map(int, input().split())) for _ in range(m)]
node = [[] for _ in range(n)]
for i in range(m):
	u, v, c = ABC[i]
	u, v = u-1, v-1
	node[u].append((v, c))
	node[v].append((u, c))
def dijk(start = 0):
    from heapq import heappop, heappush
    D = [float("INF") for _ in range(n)]
    hq = []
    heappush(hq, (0, start))
    D[start] = 0
    while hq:
        dist, now = heappop(hq)
        if D[now] < dist:
            continue
        for nxt, d in node[now]:
            if D[nxt] > D[now] + d:
                D[nxt] = D[now] + d
                heappush(hq, (D[nxt], nxt))
    return D
   
D = dijk()
ans = 0
for i in range(p):
	d, e = map(int, input().split())
	ans = max(ans, (y - D[d-1]) // e)
print(ans)