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: Accepted)

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
using Int = long long;
#define REP(i,n) for(int i=0;i<n;i++)
#define FOR(i,f,t) for(int i=f;i<t;i++)
#define P(ary) do { for(auto i:ary){ cout << i << " "; } cout << endl; } while (0)
int main()
{
    int N; cin >> N;
    vector<int> A(N), B;
    REP(i, N) cin >> A[i];
    sort(A.rbegin(), A.rend());
    int i, last = 0;
    for (i = 0; i < A.size(); i++) {
        if (A[i] == last) {
            B.push_back(A[i]);
            last = 0;
        } else {
            last = A[i];
        }
    }
    cout << (B.size() >= 2 ? (Int)B[0] * B[1] : 0) << endl;
    return 0;
}