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