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;
typedef long long ll;
ll mod = 1e9+7;
#define rep(i,n) for(int i = 0; i < (n); ++i)

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

    vector<ll> ans_prob;
    vector<ll> ans_prob2;
    for ( auto tmp_mp : mp ) {
        if ( tmp_mp.second >= 2 ) {
            ans_prob.push_back(tmp_mp.first);
        }
        if ( tmp_mp.second >= 4 ) {
            ans_prob2.push_back(tmp_mp.first);
        } 
    }


    sort(ans_prob2.begin(), ans_prob2.end());
    sort(ans_prob.begin(), ans_prob.end());

    if ( ans_prob.size() < 2 && ans_prob2.size() == 0 ) {
        cout << 0 << endl;
    }
    else if ( !ans_prob2.empty() ) {
        cout << ans_prob2[ans_prob2.size()-1] * ans_prob2[ans_prob2.size()-1] << endl;
    } 
    else {
        cout << ans_prob[ans_prob.size()-1] * ans_prob[ans_prob.size()-2] << endl;
    }
    return 0;
}