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: Wrong Answer)

#include <stdio.h>


int main(void){
	int n;
	scanf("%d",&n);
	int a[n];
	int check[n];
	for(int i=0;i<n;i++) {scanf("%d",&a[i]);check[i]=0;}
	
	int x=0;
	int y=0;//辺
	int max=0;
	int flag=0;
	int number=1;
	while(1){
		for(int i=0;i<n;i++){
			if(max<a[i]&&check[i]==0){max=a[i];}
		}
		if(max==0) {x=0;break;}
		for(int i=0;i<n;i++){
			if(max==a[i]){
				if(flag==1){x=max;}
				flag=1;check[i]=1;number++;
			}
		}
		
		if(x!=0) break;
		max=0;
		flag=0;
		number=0;
	}
	max=0;flag=0;
	if(number>=4||x==0){
		y=x;
	}
	else{
	while(1){
		for(int i=0;i<n;i++){
			if(max<a[i]&&check[i]==0){max=a[i];}
		}
		if(max==0){y=0; break;}
		for(int i=0;i<n;i++){
			if(max==a[i]){
				if(flag==1){y=max;}
				flag=1;check[i]=1;
			}
		}
		max=0;
		flag=0;
		if(y!=0) break;
	}
	}
	
	printf("%d",x*y);
	return 0;
}