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: Runtime Error)

#include<iostream>
#include<algorithm>
using namespace std;

int main(){
  int N,A[10000],H,W;
  cin >> N;

  for (int i = 0; i < N; i++){
    cin >> A[i];
  }

  sort(A, A+N, greater<int>());

  for (int i = 0; i < N; i++){

    //4本あるか判定
    if ( A[i+3] && A[i] == A[i+1] && A[i+1] == A[i+2] && A[i+2] == A[i+3]){
      if(!H) H = A[i];
      else   W = A[i];
      i += 3;
    }
    //3本あるか判定
    else if(A[i+2] && A[i] == A[i+1] && A[i+1] == A[i+2]){
      if(!H) H = A[i];
      else   W = A[i];
      i += 2;
    }

    //2本あるか判定
    if(A[i+1] && A[i] == A[i+1]){
      if(!H) H = A[i];
      else   W = A[i];
      i += 1;
    }

    //縦横が埋まっていれば面積出力
    if (H && W){
      cout << H*W << endl;
      return 0;
    }

    //デバッグ
    cout << H  << " " << A[i] << endl;
  }

  cout << 0 << endl;
  return 0;
}