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: Runtime Error)

#include<stdio.h>
#define maxn 10005
long long s[maxn];


int main()
{
	int N, a = 0, b = 0, count = 0;
	scanf("%d", &N);
	for(int i = 0; i < N; i++)
		scanf("%ld", &s[i]);
	for(int i = 0; i < N; i++)
	{
		for(int j = 0; j < i; j++)
		{
			if(s[i] == s[j])
				a = s[i];
			if(a > b)
			{
				b = a;
				break;
			}
		}
		for(int j = i + 1; j < N; j++)
		{
			if(s[i] == s[j])
				a = s[i];
			if(a > b)
				b = a;
		}
	}
/*	
		printf("%d %d\n", a, b);
*/	
	for(int i = 0; i < N; i++)
		if(s[i] == b)
			count++;
	if(count >= 4)
		printf("%d", b * b);
	else
	{
		int T = b;
		while(T--)
		{
			for(int i = 0; i < N; i++)
				if(s[i] == T)
				{
					a = s[i];
					break;
				}
			if(a < b)
				break;
		}
/*	
		printf("%d %d\n", a, b);
*/	
	}
	if(a && b)
		printf("%ld", a * b);
	else
		printf("0");
	return 0;
}