N, T = map(int, input().split())
t = list(map(int, input().split()))
K = int(input())
x = list(map(int, input().split()))
clock = [0 for _ in range(N)]
for xx in x:
  clock[xx - 1] += 1

now, c = T, 0
for i in range(N - 1):
  while now - t[i] <= 0:
    if c == 0:
      print(-1)
      exit()
    now += 10
    c -= 1
  now -= t[i]
  c += clock[i + 1]
print(sum(clock) - c)