def check(x):
    cur = T
    for i in range(N - 1):
        if cur >= 1 and i in X and x > 0:
            cur += 10
            x -= 1
        cur -= ts[i]
        if cur <= 0:
            return False
    return True

N, T = map(int, input().split())
ts = list(map(int, input().split()))
K = int(input())
X = set(map(int, input().split()))

if not check(K):
    print('-1')
    exit()

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