N = input()
sor = [int(i) for i in input().strip().split(' ')]

sor.sort(reverse=True)
h = len(sor)

i, a, b, ok = 0, None, None, True

  
while i < h-3 and h > 3:
    
    if sor.count(sor[i]) > 3:
        print(sor[i]**2)
        ok = False
    else:
        if sor[i] == sor[i+1] and a == None:
            a, j = sor[i], 0
            while i+j < h-3 and sor[i+j] == a:
                j += 1
            i += j+1
            print(i, sor[i])
            if sor[i] == sor[i+1] and b == None:
                print(a*sor[i])
                ok = False
                break
    
    i += 1

if ok:
    print(0)