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 <bits/stdc++.h>

using namespace std;

//#define int long long
#define pb push_back
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define REP(i, n) FOR(i, 0, n)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define RREP(i, n) FOR(i, n, 0);

signed main () {
  cin.tie(0);
  cout << setprecision(10);

  int n; cin >> n;
  map<int, int> a;
  int tmp;
  REP(i, n) {
    cin >> tmp;
    a[tmp]++;
  }

  int t1 = 0, t2 = 0;

  for(auto i=a.rbegin();i != a.rend();i++) {
    if(i->second >= 4) {
      t1 = i->first;
      t2 = i->first;
      break;
    } else if(i->second >= 2 && t1 == 0 && t2 == 0) {
      t1 = i->first;
    } else if(i->second >= 2) {
      t2 = i->first;
      break;
    }
  }
  cout << t1 * t2 << endl;

  
}