n,rest=map(int,input().split())
t=list(map(int,input().split()))
k=int(input())
p=set(map(int,input().split()))
need=(sum(t)-rest+1+9)//10
if need>k:
    print(-1)
    exit()
cnt=0
for i in range(n-1):
    if rest>t[i]:
        rest-=t[i]
        if i+2 in p:
            rest+=10
            cnt+=1
            if cnt==need:
                print(need)
                exit()
    else:
        print(-1)
        exit()
print(0)