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<bits/stdc++.h>
#define watch(x) cout << (#x) << " is " << (x) << endl
typedef long long ll;
using namespace std;
int static fast = [](){
  ios::sync_with_stdio(false);
  cin.tie(0); cout.tie(0); return 0;
}();
// freopen("input.txt", "r", stdin);

int main() {
    int n, val;
    cin >> n;
    map<int, int> cnt;
    for(int i = 0; i < n; i++) {
        cin >> val;
        cnt[val] += 1;
    }
    vector<ll> arr;
    for(auto p: cnt) {
        while(p.second > 1) {
            arr.push_back(p.first);
            p.second -= 2;
        }
    }
    if (arr.size() < 2)
        cout << 0 << endl;
    else
        cout << arr.back() * arr[arr.size()-2] << endl;
    return 0;
}