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;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef pair<ll, ll> P;
#define rep(i, n) for(ll i = 0; i < n; i++)
#define REP(i, a, b) for(ll i = a; i <= b; i++)
#define out(x) std::cout << x << '\n'
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define sz(x) ((int)(x).size())
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { ll g = gcd(a, b); return a / g * b; }
const int dx[4] = { 1, 0, -1,  0 };
const int dy[4] = { 0, 1,  0, -1 };
const int INF = INT_MAX / 2;
const ll  inf = LLONG_MAX / 2;
const int mod = 1000000007;
const int MAX_N = 101010;
const double PI = acos(-1);

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << fixed << setprecision(10);
	/*------------------------------*/

	ll n;
	cin >> n;
	vector<ll> a(n);
	rep(i, n) cin >> a[i];

	sort(rall(a));

	ll h = 0, w = 0;
	for (int i = 0; i < n - 1; i++) {
		if (a[i] == a[i + 1]) {
			if (h == 0) {
				h = a[i];
			}
			else if(w == 0) {
				w = a[i];
			}
			i++;
		}
	}

	out(h * w);

	return 0;
}