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 <iterator>
#include <algorithm>
#include <vector>

using namespace std;
typedef long long LL;

int main(){
  int N;
  string S1,S2;
  cin >> N;
  cin >> S1;
  cin >> S2;

  LL sum=1;

  if(N==1){
	cout << 3 << endl;
	return 0;
  }
  if(N==2){
	cout << 6 << endl;
	return 0;
  }
  if(S1[0]==S2[0] && S1[1]==S2[1]){
	sum*=12;
  }else{
	sum*=6;
  }
  
  for(int i=3;i<N;i++){
	if((S1[i-1]==S2[i-1])&&(S1[i]==S2[i])){
	  sum*=2;
	}
	else if((S1[i-1]==S2[i-1])&&(S1[i]!=S2[i])){
	  sum*=2;
	  i++;
	}
	else if((S1[i-2]==S1[i-1])&&(S1[i]==S2[i])){
	}
	else if((S1[i-2]==S1[i-1])&&(S1[i]!=S2[i])){
	  sum*=3;
	  i++;
	}
	if(sum>=1000000007){
	  sum=sum%1000000007;
	}
  }
  
  cout << sum%1000000007 << endl;

  return 0;
}