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

cnt = 0
for i in range(N-1):
    if i+1 in X:
        cnt += 1
    while cnt and T <= t[i]:
        cnt -= 1
        T += 10
    if T <= t[i]:
        print(-1)
        break
    T -= t[i]
else:
    print(K-cnt)