from heapq import heappop, heappush


N, M, P, Y = list(map(int, input().split()))

G = [[] for _ in range(N)]
for _ in range(M):
    A, B, C = list(map(int, input().split()))
    G[A - 1].append((B - 1, C))
    G[B - 1].append((A - 1, C))

S = {}
for _ in range(P):
    D, E = list(map(int, input().split()))
    S[D - 1] = E


def dijkstra(s):
    dist = [float("inf")] * N
    dist[s] = 0
    hq = [(0, s)]
    while hq:
        d, u = heappop(hq)
        if dist[u] < d:
            continue
        for v, w in G[u]:
            if dist[v] > dist[u] + w:
                dist[v] = dist[u] + w
                heappush(hq, (dist[v], v))
    return dist


dist = dijkstra(0)
ans = 0
for v in S.keys():
    ans = max(ans, (Y - dist[v]) // S[v])

print(ans)