n,T=map(int,input().split())
t=list(map(int,input().split()))
K=int(input())
x=list(map(int,input().split()))
p=[0]*n
for v in x:
  p[v-1]=1
ok=K+1
ng=-1
while ok-ng>1:
  m=(ok+ng)//2
  now=T
  k=m
  for i in range(n-1):
    now-=t[i]
    if now<=0:
      break
    if k>0 and p[i+1]:
      now+=10
      k-=1
  if now>0:
    ok=m
  else:
    ng=m
print(ok if 0<=ok<=K else -1)