def solve():
    N, T = map(int, input().split())
    t = list(map(int, input().split()))
    K = int(input())
    x = list(map(int, input().split()))
    tmp = [0]
    p = [0] * N
    for i in x:
        p[i - 1] += 1
    for i in range(1, N):
        p[i] += p[i - 1]
    for i in range(N - 1):
        T -= t[i]
        while T <= 0:
            tmp[-1] += 1
            T += 10
        tmp.append(tmp[-1])
    for i in range(N):
        if p[i] < tmp[i]:
            print(-1)
            return
    print(tmp[-1])


solve()