import heapq

N,M,P,Y=(int(x) for x in input().split())
d={}
for i in range(1,N+1):
    d[i] = []

for i in range(M):
    A,B,C=(int(x) for x in input().split())
    d[A].append([B,C])
    d[B].append([A,C])

shop={}
for i in range(P):
    D,E=(int(x) for x in input().split())
    shop[D] = E

inf=float("inf")
money_lose=[inf]*N
money_lose[0]=0
L=[[1,0]]
heapq.heapify(L)
while L:
    a=heapq.heappop(L)
    u=a[0]
    c=a[1]
    if c > money_lose[u-1]:
        continue

    for v, cost in d[u]:
        if money_lose[v-1] > c + cost:
            money_lose[v-1] = c + cost
            heapq.heappush(L,[v, c+cost])

ans=0
for i in range(N):
    if i+1 in shop:
        ans = max(ans, (Y-money_lose[i])//shop[i+1])

print(ans)