# coding: utf-8
# Your code here!

import sys
readline = sys.stdin.readline
read = sys.stdin.read

n,*a = [int(i) if i != "-1" else 0 for i in read().split()]

from math import gcd
from functools import reduce 

g = reduce(gcd,a)

if g: print(g*g)
else: print(-1)