# -*- coding: utf-8 -*-
#апрыерка нечётных чисео на простоту
def prime(n):
   if n % 2 == 0:
        return n == 2
   d = 3
   while d * d <= n and n % d != 0:
       d += 2
   return d * d > n

n = int(input())
k = int(input())
m = [0 for i in range(k)]
#заполненние массива делителями из условия
for i in range(k):
    m[i] = int(input())

ms = []
for i in range(k):
    if (prime(m[i])) and not (m[i] in ms):
        ms.append(m[i])
ms = sorted(ms)
#f101 - массив с первыми 101 простыми числами
f101 = []
counter = 2
while len(f101) < 101:
    if prime(counter):
        f101.append(counter)
    counter += 1

answer = -1
for i in range(len(ms)):
    if ms[i] == 1:
        answer = 2
        break
    if ms[i] != f101[i]:
        answer = f101[i] ** 2
        break
if (answer == -1) and (ms[len(ms)-1]**2 < n):
    answer = f101[len(ms)]**2
print(answer)