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"
#define fi first
#define se second
#define all(x) x.begin(), x.end()
using namespace std;
using ull = unsigned long long;
using ll = long long;
struct __INIT{__INIT(){cin.tie(nullptr);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}} __init;

int main(){
    int N;
    cin >> N;
    map<ll, int> count;
    for(int i = 0; i < N; i++){
        ll A;
        cin >> A;
        count[A]++;
    }
    vector<pair<ll,int>> v;
    for(auto p : count){
        if(p.se >= 2){
            v.emplace_back(p.fi, p.se);
        }
    }
    sort(all(v), greater<>());
    ll S;
    if(v.size() == 1){
        if(v[0].se >= 4) S = v[0].fi * v[0].fi;
        else S = 0;
    }else if(v.empty()) S = 0;
    else{
        if(v[0].se >= 4) S = v[0].fi * v[0].fi;
        else S = v[0].fi * v[1].fi;
    }
    cout << S << endl;
}