(n,t),ts,(k,),x=[[*map(int,s.split())]for s in open(0)]
x={i-1 for i in x}
sk=ans=0
for i in range(n-1):
  t-=ts[i]
  if t+sk*10<=0:
    exit(print(-1))
  while t<=0:
    sk-=1
    t+=10
    ans+=1
  # print(i,ts[i],sk,t,ans)
  if i+1 in x:
    sk+=1
print(ans)