import collections,sys,math,functools,operator,itertools,bisect,heapq,decimal,string,time,random
#sys.setrecursionlimit(10**9)
#sys.set_int_max_str_digits(0)
#input = sys.stdin.readline
#n = int(input())
#
#alist = []
#s = input()
n,t = map(int,input().split())
a = list(map(int,input().split()))
k = int(input())
#for i in range(n):
#    alist.append(list(map(int,input().split())))
b = set(map(lambda x:int(x)-1,input().split()))
l,r = 0,k
for _ in range(20):
    mid = (l+r)//2
    now = 0
    cnt = t
    mm = mid
    f = 1
    for i in range(n-1):
        cnt -= a[i]
        if cnt <= 0:
            f = 0
            break
        if mm > 0 and i+1 in b:
            mm -= 1
            cnt += 10
    if f == 0:
        l = mid
    else:
        r = mid

    
for mid in range(r-3,r+3):
    if mid < 0:
        continue
    now = 0
    cnt = t
    mm = mid
    f = 1
    for i in range(n-1):
        cnt -= a[i]
        if cnt <= 0:
            f = 0
            break
        if mm > 0 and i+1 in b:
            mm -= 1
            cnt += 10
    if f == 1:
        exit(print(mid))
print(-1)