n, T_ = map(int, input().split())
T = list(map(int, input().split()))
k = int(input())
X = list(map(int, input().split()))

l = -1
r = k + 1


def ok(x):
    a = T_
    xp = 0
    pos = 1
    for t in T:
        a -= t
        if a <= 0:
            return False
        pos += 1
        if xp < x and X[xp] == pos:
            a += 10
            xp += 1

    return True


while r - l > 1:
    mid = (l + r) // 2
    if ok(mid):
        r = mid
    else:
        l = mid

if r == k + 1:
    print(-1)
else:
    print(r)