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: Wrong Answer)

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

int main(void){
  long long N;
  cin >> N;
  vector<long long> a(N);
  for (long long i = 0;i < N + 1; i++){
    a[i] = 0;
  }

  for (long long i = 0;i < N; i++){
    long long tmp;
    cin >> tmp;
    if(tmp >= 0 && tmp < N){
      a[tmp]++;
    }
  }
  long long width,height;
  width = 0;
  height = 0;
  for (long long  i = 0;i < N; i++){
    if (a[i] >= 2 && width < i + 1 && width <= height){
      width = i;
      a[i] -= 2;
    }
    if (a[i] >= 2 && height < i + 1 && height <= width){
      height = i;
      a[i] -= 2;
    }
  }
  cout << width * height << endl;
  return 0;
}