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 <cstdio>
#include <cstring>
#include <cmath>
#include <map>
#include <stack>
#include <algorithm>
using namespace std;
const int Max = 2e5;
 
int n, a[Max + 5], l, r;
long long ans;
map<int, int> cnt;

bool cmp(int m, int n){
	return m > n;
}
 
int main()
{
	scanf ("%d", &n);
	for (int i = 1; i <= n; i++){
		scanf ("%d", &a[i]);
		cnt[a[i]]++;
	}
	l = 0;
	r = 0;
	sort(a + 1, a + n + 1, cmp);
	for (int i = 1; i <= n; i++){
		if (cnt[a[i]] >= 2){
			if (!l)	l = a[i], cnt[a[i]]-=2;
			else r = a[i];
			if (l && r)	break;
		}
	}
	ans = l * r;
	printf ("%I64d\n", ans);
}