import sys, math
from itertools import permutations
input = sys.stdin.readline

n = int(input())
a = list(map(int, input().split()))

if(n >= 4):
	print(0)
	exit(0)

def f(x, y):
	if(math.gcd(x, y) >= 2):
		return 0
	return (x - 1) * (y - 1)

ans = math.inf
for p in permutations(a):
	t = f(p[0], p[1])
	if(n == 3):
		t = f(t, p[2])
	ans = min(ans, t)

if(ans > 10 ** 18):
	ans = -1
print(ans)