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 cnt < -((t-1)//10):
        exit(print(-1))
    if t <= 0:
        l = -((t-1)//10)
        cnt -= l
        t += l*10
    if x and x[-1] == i+2:
        cnt += 1
        x.pop()
print(k - cnt)