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;
typedef pair<int, int> P;
#define rep(i,n) for(int i=0;i<n;++i)
#define MOD 1000000007

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

    int L1=0, L2;
    for(auto u:mp){
        if(u.second>=2 && u.first>L1){
            L1 = u.first;
        }
    }
    mp[L1] -= 2;
    // cout << L2 << endl;
    for(auto u:mp){
        if(u.second>=2 && u.first>L2){
            L2 = u.first;
        }
    }
    cout << (ll)L1*L2 << endl;
    // printf("%d\n", N);
    return 0;
}