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;
using ll = long long;
#define REP(i,m,n) for(int i=(int)(m);i<(int)(n);++i)
#define rep(i,n) REP(i,0,n)
#define all(x) (x).begin(),(x).end()
#define out(y,x,h,w) (y)<0||(x)<0||(y)>=(h)||(x)>=(w)
constexpr int INF = 2000000000;
constexpr ll mod = 1000000007;

template<class T>inline bool chmin(T& a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}
template<class T>inline bool chmax(T& a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
}

int main() {

	cin.tie(nullptr);
	ios::sync_with_stdio(false);

	int N; cin >> N;
	map<ll,ll>A;
	rep(i, N) {
		ll a;cin >> a;
		A[a]++;
	}
	ll ans = 0;
	for (auto p : A) {
		if (p.second >= 4) {
			chmax(ans, p.first* p.first);
		}
	}
	ll x = 0;int cnt = 0;
	for (auto p : A) {
		if (cnt == 0&&p.second>=2) {
			x = p.first;
			++cnt;
			continue;
		}
		if (p.second >= 2) {
			chmax(ans, p.first* x);
			x = p.first;
		}
	}
	cout << ans << "\n";
	return 0;
}