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: Accepted)

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

int main(int argc, char *argv[]) {
  int n;
  cin >> n;
  map<int, int> mp;
  ll ans = 0, mx = 0;
  for (int i = 0; i < n; i++) {
    int t;
    cin >> t;
    mp[t]++;
  }
  for (auto e : mp) {
    if (e.second >= 2) {
      ans = max(ans, mx * e.first);
      mx = e.first;
    }
    if (e.second >= 4) {
      ans = max(ans, (ll)e.first * e.first);
    }
  }
  cout << ans << endl;
  return 0;
}