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>

#define rep(i, n) for (int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;
const int INF = 1e9;

int main() {
  int n, a, max = 0, ans;
  cin >> n;
  map<int, int, greater<int>> mp;
  rep(i, n) {
    cin >> a;
    mp[a]++;
  }

  for (auto x : mp) {
    if (x.second >= 2)
      if (max == 0)
        max = x.first;
      else {
        ans = x.first * max;
        break;
      }
    else if (x.second >= 4) {
      if (max == 0) {
        ans = x.first * x.first;
        break;
      } else {
        ans = x.first * max;
        break;
      }
    }
  }
  cout << (max ? ans : 0) << endl;
  return 0;
}