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 PyPy3 (2.4.0) (judge verdict: Accepted)

import sys
def input(): return sys.stdin.readline().strip()

def resolve():
    n=int(input())
    l=list(map(int,input().split()))
    from collections import Counter
    c=Counter(l)# ()にカウンターの対象のリストの変数名
    ans=[]
    for key,value in c.items():
        if value>=4:
            ans.append(key)
            ans.append(key)
        elif value>=2:
            ans.append(key)
    ans.sort(reverse=True)
    if ans:
        print(ans[0]*ans[1])
    else:
        print(0)
resolve()