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 <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <map>
using namespace std;
#define rep(i,n) for (int i=0; i<(int)(n); i++)
#define ct(string) cout << string << endl

int main() {
    int n, m1=0, m2=0;
    cin >> n;
    vector<int> vec(n);
    rep(i,n) cin >> vec.at(i);
    sort(vec.begin(), vec.end());
    reverse(vec.begin(), vec.end());
    int current=vec.at(0), c=0, sum=0;
    rep(i,n) {
        if (current != vec.at(i)) {
            if (c >= 4) {m1=vec.at(i-1); m2=m1; c=0; break;}
            if (c >= 2) {m2=m1; m1=vec.at(i-1); sum+=2;}
            if (sum == 4) {c=0; break;}
            c = 0;
            current = vec.at(i);
        }
        c++;
    }
    if (c >= 4) {m1=vec.at(n-2); m2=m1;}
    if (c >= 2) {m2=m1; m1=vec.at(n-2);}
    ct((int64_t) m1*m2);
}