N,T = list(map(int,input().split()))
t = list(map(int,input().split()))
K = int(input())
x = set(map(lambda x:int(x)-1,input().split()))

def check(n):
    time = T
    get = 0
    for i in range(N-1):
        time -= t[i]
        if(time <= 0):return False
        if(i+1 in x and get < n):
            get += 1
            time += 10
    return True

ng,ok = -1,K+1
while(ok - ng > 1):
    mid = (ok+ng)//2
    if(check(mid)):
        ok = mid
    else:
        ng = mid
print(ok if ok != K+1 else -1)