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

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

    if sum(A) == -N:
        print(-1)
        exit()

    B = []
    for a in A:
        if a != -1:
            B.append(a)
    B.sort()
    g = B[0]
    for b in B[1:]:
        g = gcd(g, b)
    print(g**2)


if __name__ == '__main__':
    main()