count = int(input())
val = {}
for i in range(count):
    temp = int(input())
    if temp % 2 == 1:
        val[i] = temp

def find_min(val):

    res = []

    for key1 in val:
        for key2 in val:
            if key2 - key1 > 5:
                res.append(val[key1]*val[key2])

    if res:
        return min(res)
    else:
        return -1

print (find_min(val))