INF = float("inf")

N, T = map(int, input().split())
t_list = list(map(int, input().split()))

K = int(input())
x_set = set(map(int, input().split()))

def check(n):
    time = T
    cnt = 0
    for i, t in enumerate(t_list, start=1):
        if cnt < n and i in x_set:
            time += 10
            cnt += 1
        time -= t
        if time <= 0:
            return False
    return True

ok, ng = K + 1, -1
while abs(ok - ng) > 1:
    mid = (ok + ng) // 2
    if check(mid):
        ok = mid
    else:
        ng = mid

print(ok if ok <= K else -1)