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<stdio.h>
#include<stdlib.h>
#include<algorithm>
#include<bitset>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<functional>
#include<list>
#include<map>
#include<iomanip>
#include<iostream>
#include<queue>
#include<set>
#include<stack>
#include<string>
#include<vector>
#include<algorithm>
#include<memory.h>
using namespace std;
int main(){
	long long N,a[100005],b[100005],j=1;
	memset(a,0,sizeof(a));
	memset(b,0,sizeof(b));
	cin>>N;
	for(int i=1;i<=N;i++)
		cin>>a[i];
	sort(a+1,a+N+1);
	for(int i=N;i>=1;i-=2){
		if(a[i]==a[i-1]){
			b[j]=a[i];
			j++;
		}
	}
	if(b[1]==0||b[2]==0){
		cout<<0<<endl;
		return 0;
	}
	cout<<b[1]*b[2]<<endl;
	return 0;
}