from collections import deque

n,m,p,y=map(int,input().split())
edge=[list(map(int,input().split())) for _ in range(m)]
shop=[list(map(int,input().split())) for _ in range(p)]

E=[[] for _ in range(n+1)]
for a,b,c in edge:
  E[a].append([b,c])
  E[b].append([a,c])
ds=dict()
for d,e in shop:
  ds[d]=e

D=[0]*(n+1)
D[1]=y
q=deque([1])
while q:
  x=q.popleft()
  for e,c in E[x]:
    if D[e]<max(0,D[x]-c):
      D[e]=max(0,D[x]-c)
      q.append(e)
ans=0
for i in range(1,n+1):
  if i in ds:
    ans=max(ans,D[i]//ds[i])
print(ans)