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

items = 0
ans = 0
j = 0
for i in range(N-1):
  T -= t[i]
  
  while T <= 0 and items:
    T += 10
    items -= 1
    ans += 1
    
  if T <= 0:
    ans = -1
    break
  
  if j < K and x[j] == i+2:
    j += 1
    items += 1
  
print(ans)