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 <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
#define REP(var, a, b) for (int var = (a); var < (b); var++)
#define rep(var, n) for (int var = 0; var < (n); ++var)
#define ALL(c) (c).begin(), (c).end()
#define rALL(c) (c).rbegin(), (c).rend()

int main() {
  //
  int n;
  cin >> n;
  map<ll, ll, greater<ll>> m;
  rep(i, n) {
    ll a;
    cin >> a;
    if (m.find(a) == m.end())
      m[a] = 1;
    else
      m[a]++;
  }
  ll len1 = 0;
  for (auto& x : m) {
    if (x.second >= 4) {
      if (len1 == 0) {
        cout << x.first * x.first << endl;
        return 0;
      } else {
        cout << len1 * x.first << endl;
        return 0;
      }
    } else if (x.second >= 2) {
      if (len1 == 0) {
        len1 = x.first;
      } else {
        cout << len1 * x.first << endl;
        return 0;
      }
    }
  }
  cout << 0 << endl;
  return 0;
}