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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
#define IO ios::sync_with_stdio(0);cin.tie(0)
#define endl '\n'
#define MAXN maxn

using namespace std;

int N;
set<int> s;
long long mx, mx2;

signed main()
{
  IO;
  cin >> N;
  for (int i = 0, t; i < N; i++) {
    cin >> t;
    if (s.find(t) != s.end()) {
      if (t > mx)
        mx2 = mx, mx = t;
      else if (t > mx2) 
        mx2 = t;
      s.erase(t);
    }
    else 
      s.insert(t);
  }
  cout << mx * mx2 << endl;
  return 0;
}