def main():
    import sys
    from math import gcd
    input = sys.stdin.readline

    N = int(input())
    A = list(map(int, input().split()))

    g = 0
    for a in A:
        if a != -1:
            g = gcd(g, a)
    if g != 0:
        print(g**2)
    else:
        print(-1)


if __name__ == '__main__':
    main()