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)

# -*- coding: utf-8 -*-

# AtCoder Beginner Contest
# Problem C

if __name__ == '__main__':
    from collections import Counter

    n = int(input())
    a = Counter(list(map(int, input().split())))
    sorted_a = sorted(a.items(), key=lambda x: (x[0], x[1]), reverse=True)
    squares = [key for key, value in sorted_a if value >= 4]
    rectangles = [key for key, value in sorted_a if value >= 2]

    if (len(squares) >= 1) and (len(rectangles) >= 2):
        print(max(squares[0] ** 2, rectangles[0] * rectangles[1]))
    elif (len(squares) >= 1) and (len(rectangles) < 2):
        print(squares[0] ** 2)
    elif len(rectangles) >= 2:
        print(rectangles[0] * rectangles[1])
    else:
        print(0)