n,m,p,Y=map(int,input().split())
e=[[] for i in range(n)]
for i in range(m):
  a,b,c=map(int,input().split())
  a-=1
  b-=1
  e[a]+=[(b,c)]
  e[b]+=[(a,c)]
X=10**20
h=[X]*n
for i in range(p):
  u,v=map(int,input().split())
  u-=1
  h[u]=v
from heapq import heappush,heappop
v=[X]*n
s=0
v[s]=0
q=[(v[s],s)]
while len(q)>0:
  sc,sp=heappop(q)
  if sc>v[sp]:
    continue
  for tp,tc in e[sp]:
    if v[tp]>sc+tc:
      v[tp]=sc+tc
      heappush(q,(v[tp],tp))
ans=0
for i in range(n):
  ans=max(ans,(Y-v[i])//h[i])
print(ans)