n, T = map(int, input().split())
t = list(map(int, input().split()))
K = int(input())
x = list(map(int, input().split()))
X = set(x)
ans = cnt = 0
for i in range(n-1):
    if T > 0:
         T -= t[i]
         while 0 < cnt and T <= 0:
             cnt -= 1
             T += 10
             ans += 1
         if i + 2 in X:
             cnt += 1
         if T <= 0:
             print(-1)
             break
else: 
    print(ans)