import sys, time, random
from collections import deque, Counter, defaultdict
def debug(*x):print('debug:',*x, file=sys.stderr)
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 61 - 1
mod = 998244353

n, t = mi()

d = li()

k = ii()

x = li()

clock = [0] * n

for i in range(k):
    clock[x[i] - 1] += 1


cnt = 0
ans = 0
for i in range(n - 1):
    if clock[i] > 0:
        cnt += 1
    while t - d[i] <= 0:
        if cnt == 0:
            print(-1)
            exit()
        t += 10
        ans += 1
        cnt -= 1
    t -= d[i]
    
print(ans)