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)

def c_make_a_rectangle(N, A):
    from collections import Counter
    stick = Counter(A)  # ある長さの棒が何本あったか
    rectangle = []
    for length, num in sorted(stick.items(), reverse=True):  # 長いもの順に見る
        rectangle.extend([length] * (num // 2))  # 2本組を何個作れるか
    ans = 0 if len(rectangle) <= 1 else rectangle[0] * rectangle[1]
    return ans

N = int(input())
A = [int(i) for i in input().split()]
print(c_make_a_rectangle(N, A))