n,t=map(int,input().split())
L=list(map(int,input().split()))
k=int(input())
A=list(map(int,input().split()))
S=set(A)
cnt=0
ans=0
for i in range(n-1):
	T-=L[i]
	while 0<cnt and T<=0:
		cnt-=1
		T+=10
		ans+=1
	if i in S:
		cnt+=1
	if T<=0:
		exit(print(-1))
print(ans)