def main():
  n, t, = map(int, input().split())
  T = list(map(int, input().split()))
  input()
  X = list(map(int, input().split()))

  X.append(10**18)
  X.reverse()
  cnt = 0
  for i, ti in enumerate(T, 2):
    t -= ti
    if t <= 0:
      print(-1)
      return
    if i == X[-1]:
      cnt += 1
      t += 10
      X.pop()

  cnt -= min((t-1) // 10, cnt)
  print(cnt)

main()