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<cstdio>
#include<cstring>
#include<algorithm>
#include<string>
#include<cmath>
#include<ctime>
#include<queue>
#include<map>
#define rint register int
#define debug(x) cerr<<#x<<"="<<x<<endl
#define fgx cerr<<"-------------"<<endl
typedef long long ll;
using namespace std;
map<int,bool> mp,vis;
int a[100001],b[100001];
int main()
{	int n,tot=0,maxn=0; cin>>n;
	for(rint i=1;i<=n;i++)
	{	cin>>a[i];
		if(mp[a[i]]&&!vis[a[i]])
		vis[a[i]]=true,b[++tot]=a[i];
		mp[a[i]]=true;
	}
	for(rint i=1;i<=tot;i++) maxn=max(b[i],maxn);
	int ans=maxn; bool bb=false; maxn=0;
	for(rint i=1;i<=tot;i++)
	{	if(b[i]==ans&&bb==false) {bb=true;continue;}
		maxn=max(maxn,b[i]);
	}
	cout<<ans*maxn<<endl;
 	return 0;
}