import sys
input = sys.stdin.readline

N,T=map(int,input().split())
TL=list(map(int,input().split()))

K=int(input())
X=set(map(int,input().split()))

canuse=0
ANS=0



for i in range(N-1):
    T-=TL[i]

    while T<=0 and canuse>0:
        T+=10
        canuse-=1
        ANS+=1

    if T<=0:
        print(-1)
        exit()

    if i+2 in X:
        canuse+=1

print(ANS)