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

int main(void){
  vector<int> v;
  int cnt[10010],num[10010];
  int n,m,l,x,y;
  
  cin >> n;
  for (int i=0; i<n; i++){
    int t; cin >> t;
    v.push_back(-t);
  }
  sort(v.begin(), v.end());
  m = l = 0;
  for (int i=0; i<n; i++){
    if (v[i]!=l){
      cnt[m]=1;
      num[m]=v[i];
      m++;
    } else {
      cnt[m-1]++;
    }
    l = v[i];
  }
  x = y = 0;
  for (int i=0; i<m; i++){
//    cout << cnt[i] << " " << num[i] << endl;
    if (cnt[i]>=4 && x==0 && y==0) x=y=num[i];
    else if (cnt[i]>=2 && x==0) x=num[i];
    else if (cnt[i]>=2) y=num[i];
    if (x*y) break;
  }
  cout << (x*y) << endl;
  return 0;
}