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: Runtime Error)

/*
これを入れて実行
g++ code.cpp
./a.out
 */

#include <iostream>
#include <vector>
#include <string>
#include <queue>
#include <algorithm>
#include <utility>
#include <set>
#include <map>
#include <cmath>
#include <tuple>
#include <iomanip>

using namespace std;
typedef long long ll;
typedef long double ld;

int dy4[4] = {-1, 0, +1, 0};
int dx4[4] = {0, +1, 0, -1};
int dy8[8] = {-1, -1, 0, 1, 1, 1, 0, -1};
int dx8[8] = {0, 1, 1, 1, 0, -1, -1, -1};

const long long INF = 1LL << 60;
const ll MOD = 1e9 + 7;

bool greaterSecond(const pair<int, int>& f, const pair<int, int>& s){
    return f.second > s.second;
}

int gcd(int a, int b){
	if (b == 0)return a;
	return gcd(b, a % b);
}

ll nCm(ll n, ll m){
    if(m == 0 || m == n){
        return 1;
    } else if(m == 1){
        return n;
    }
    return (nCm(n - 1, m) + nCm(n - 1, m - 1));
}

//-----------------------ここから-----------

int main(void){
    int n;
    cin >> n;
    vector<int> a(n);
    for(int i = 0; i < n; i++) cin >> a[i];

    int backet[100100] = {0};
    for(int i = 0; i < n; i++){
        backet[a[i]]++;
    }

    vector<int> l;

    for(int i = 0; i < 100100; i++){
        if(backet[i] > 1){
            l.push_back(i);
        }
    }

    sort(l.begin(), l.end(), greater<int>());
    if(l.size() < 3){
        cout << 0 << endl;
        return 0;
    }

    cout << l[0] * l[1] << endl;
}