def test():
	MassR = [True] * 5000
	for i in range(2,5000):
		if MassR[i] == True:
			for j in range(i*i,5000,i):
				MassR[j] = False
	MassProst = []
	for i in range(2,5000):
		if len(MassProst) > 300:
			break
		else:
			if MassR[i] == True:
				MassProst.append(i)
	rep = True
	n = int(input())
	k = int(input())
	a = [0 for i in range(k)]
	ans = -1
	for i in range(k):
		a[i] = int(input())
		if(a[i] == 1):
			print("2")
			return 0
	for i in range(0,len(MassProst)):
		rep = False
		for j in range(k):
			if MassProst[j] == a[j]:
				rep = True
		if rep == False:
			ans = MassProst[i] * MassProst[i]
			break
	if ans <= n:
		print(ans)
	else:
		print("-1")
test()