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 <iostream>
#include <vector>
#include <stack>
#include <queue>
#include <algorithm>
#include <cstdlib>
#include <cmath>
#include <numeric>
#include <map>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define repi(i, start, end) for(int i = start; i < end; i++)
#define all(x) (x).begin(), (x).end()
#define debug(x) cout << (x) << endl;
typedef long long ll;
const int MAX = 999999999;

bool comp(const pair<ll, ll> &a, const pair<ll, ll> &b){
    return a.first > b.first;
}

int main(){
    int n;
    cin >> n;

    map<ll, ll> len_cnt;
    rep(i, n){
        int a;
        cin >> a;
        len_cnt[a] ++;
    }

    vector<pair<ll, ll>> len_cnt_vec;
    for(auto itr = len_cnt.begin(); itr != len_cnt.end(); itr ++){
        len_cnt_vec.push_back(pair<ll, ll>(itr->first, itr->second));
    }

    sort(all(len_cnt_vec), comp);

    bool flag = false;
    if(len_cnt_vec[0].second >= 2){
        cout << len_cnt_vec[0].first * len_cnt_vec[0].first << endl;
        flag = true;
    }


    rep(i, len_cnt_vec.size()-1){
        if(!flag && len_cnt_vec[i].second >= 2 && len_cnt_vec[i+1].second >= 2){
            cout << len_cnt_vec[i].first * len_cnt_vec[i+1].first << endl;
            flag = true;
        }
    }

    if(!flag) cout << 0 << endl;
}