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>
using namespace std;
typedef long long ll;
int main()
{
    ll n;
    scanf("%lld", &n);
    map<ll, ll> mp;
    while (n--) {
        ll x;
        scanf("%lld", &x);
        mp[x]++;
    }
    ll x = 0, y = 0;
    for (auto it = mp.rbegin(); it != mp.rend(); it++) {
        if (!x && it->second >= 2) {
            x = it->first;
            mp[x] -= 2;
        }
        if (!y && it->second >= 2) {
            y = it->first;
            mp[y] -= 2;
            break;
        }
    }
    ll ans = x * y;
    printf("%lld\n", ans);
}