n,now=map(int,input().split())
t=list(map(int,input().split()))
k=int(input())
x=set(map(int,input().split()))
cnt=len(x)
through=0
nxt=now
for i in range(n-1):
    now-=t[i]
    nxt-=t[i]
    if now<1:
        if through*10+now<1:
            exit(print(-1))
        else:
            tmp=now+through*10
            through,now=divmod(tmp,10)
            if now==0:
            	now+=10
            	through-=1
    if i+2 in x:through+=1
    
    #print(now,through)
print(cnt-through)