"""

"""

import sys

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

now = T
keep = 0
ans = 0

for i in range(N-1):

    T -= t[i]
    if T <= 0:
        cnt = min(keep , 1-T//10)
        ans += cnt
        T += cnt * 10
        keep -= cnt

        if keep < 0 or T <= 0:
            print ("-1")
            sys.exit()
        
    if i+2 in x:
        keep += 1

print (ans)