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 <map>
#define F first
#define S second
using namespace std;
long long n, a, s;
map<int,int> mp;
map<int,int>::reverse_iterator it;
int main() {
	int i;
	scanf("%lld", &n);
	while(n--) scanf("%lld", &a), mp[a]++;
	for(it=mp.rbegin(); it!=mp.rend(); it++) {
		if(it->S>=4) {
			if(s>0) s *= it->F;
			else s = it->F * it->F;
			break;
		}
		else if(it->S>=2) {
			if(s>0) {s *= it->F; break;}
			else s = it->F;
		}
	}
	printf("%lld", s);
	return 0;
}