from collections import defaultdict,deque
import heapq
N,M,P,Y = list(map(int,input().split()))
edge = defaultdict(list)
for _ in range(M):
    a,b,c = list(map(int,input().split()))
    a -= 1;b -= 1
    edge[a].append((c,b))
    edge[b].append((c,a))

shop = []
for _ in range(P):
    d,e = list(map(int,input().split()))
    shop.append((d-1,e))

q = [(Y,0)]
money_s = [float("-inf") for _ in range(N)]
while(q):
    money,now = heapq.heappop(q)

    if(money_s[now] >= money):continue
    money_s[now] = money

    for c,a in edge[now]:
        if(money_s[a] >= money - c):continue
        heapq.heappush(q,(money-c,a))

ans = float("-inf")
for i in range(P):
    d,e = shop[i]
    ans = max(ans,money_s[d]//e)
print(ans)