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 (GCC 5.4.1) (judge verdict: Accepted)

#include <stdio.h>
#include <stdlib.h>

int compare( const int *v1, const int *v2 ){
	return *v1 - *v2;
}

int main(){
	
	int n, i;
	int a[100000];
	
	scanf( "%d", &n );
	for( i = 0; i < n; i++ ){
		scanf( "%d", &(a[i]) );
	}
	qsort( a, n, sizeof(int), 
		(int (*)(const void *, const void *))compare );

	long long int edge1 = -1;
	long long int edge2 = -1;
	i = n-1;
	while( i > 0 ){
		if( a[i] == a[i-1] ){
			if( edge1 == -1 ){
				edge1 = (long long int)a[i];
				i = i - 2;
				
			}else{
				edge2 = (long long int)a[i];
				i = 0;
			}
		}else{
			i = i - 1;
		}
	}
	
	if( edge2 == -1 ){
		printf("0");
		
	}else{
		printf("%lld", edge1 * edge2 );
		
	}
	
	return 0;
	
}