n, t = map(int, input().split())
a = list(map(int, input().split()))
k = int(input())
x = list(map(int, input().split()))
x.reverse()
cnt = 0
for i in range(n-1):
    t -= a[i]
    if t <= 0 and -((t-1)//10) <= cnt:
        cnt -= -((t-1)//10)
        t += 10*-((t-1)//10)
    if t <= 0 and cnt == 0:
        exit(print(-1))
    if x[-1] == i+1:
        cnt += 1
        x.pop()

if t <= 0 and -((t-1)//10) <= cnt:
    cnt -= -((t-1)//10)
    t += 10*-((t-1)//10)
if t <= 0 and cnt == 0:
    exit(print(-1))
print(k-cnt)