Problem:
Score : 300 points

Problem Statement

We have N sticks with negligible thickness.
The length of the i-th stick is A_i.

Snuke wants to select four different sticks from these sticks and form a rectangle (including a square), using the sticks as its sides.
Find the maximum possible area of the rectangle.

Constraints

4 \leq N \leq 10^5

1 \leq A_i \leq 10^9

A_i is an integer.

Input

Input is given from Standard Input in the following format:

N
A_1 A_2 ... A_N

Output

Print the maximum possible area of the rectangle.
If no rectangle can be formed, print 0.

Sample Input 1

6
3 1 2 4 2 1

Sample Output 1

2

1 \times 2 rectangle can be formed.

Sample Input 2

4
1 2 3 4

Sample Output 2

0

No rectangle can be formed.

Sample Input 3

10
3 3 3 3 4 4 4 5 5 5

Sample Output 3

20

Solution in Python (3.4.3) (judge verdict: Accepted)

import collections

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

cnt = collections.Counter(A)
k2 = []
k4 = []
for k in cnt:
    if cnt[k] >= 2:
        k2.append(k)
    if cnt[k] >= 4:
        k4.append(k)
ans = 0
k2.sort()
k4.sort()
if len(k4) >= 1:
    ans = max(ans, k4[-1] * k4[-1])
if len(k2) >= 2:
    ans = max(ans, k2[-1] * k2[-2])
print(ans)