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<climits>
#include<math.h>
#include<vector>
#include<algorithm>
#include<cstdio>
#include<stdio.h>
#include <string>
#include <complex>
#include <functional>
using namespace std;
//typedef pair<string,string> P;
//double dat[100][100];
//int dp[6][1010];//動的計画法
//int prime[10000001];
//char str[1010][1010];
//vector<pair<int,int> > pc[100001];
//int ABS(int a){return max(a,-a);}
//long long num[100000];
int main(){
	long long n;
	cin>>n;
	long long a[n];
	for(int i=0;i<n;i++) cin>>a[i];
	sort(a,a+n);
	long long t1=0,t2=0,p=0,q=0;
	for(int i=n-1;i>=1;i--){
		if(a[i]==a[i-1]){
          	if(p==0) p=a[i];
          	else{
				if(t1==0) {
                  t1=p;
                  p=0;
                }
				else if(t2==0){
					t2=a[i];
					break;
				}
            }
		}
	}
	cout<<t1*t2<<endl;
	return 0;
}