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 <vector>
#include <algorithm>
using namespace std;
int main(){
    int n,b;scanf("%d",&n);
    vector<int> a;
    for(int i=0;i<n;i++){
        scanf("%d",&b);
        a.push_back(b);
    }
    sort(a.rbegin(),a.rend());
    int ans[2]={};
    int index=0;
    auto itr = a.begin();
    b = *itr;itr++;
    while(itr!=a.end()&&index<2){
        if(*itr==b){
            ans[index]=b;
            index++;
            b = 0;
        }else b = *itr;
        itr++;
    }
    printf("%d\n",ans[0]*ans[1]);
    return 0;
}