N, T = map(int, input().split())
t = list(map(int, input().split()))
K = int(input())
x = list(map(int, input().split()))
x = [a-1 for a in x]

x.reverse()
cnt = 0
now = T
for i in range(N-1):
    while now - t[i] <= 0:
        if len(x) == 0:
            print(-1)
            exit()
        xx = x.pop()
        if xx <= i:
            cnt += 1
            now += 10
        else:
            print(-1)
            exit()
    now -= t[i]
print(cnt)