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: Wrong Answer)


// Problem : C - Make a Rectangle
// Contest : AtCoder Regular Contest 081
// URL : https://atcoder.jp/contests/arc081/tasks/arc081_a
// Memory Limit : 256 MB
// Time Limit : 2000 ms
// Powered by CP Editor (https://github.com/cpeditor/cpeditor)

#include "bits/stdc++.h"
using namespace std;
using ll = long long;
#define size(x) (int)x.size()

const int N = 1e6 + 1, mod = 1e9 + 7, inf = 2e9;

int n;
vector<int> v;
map<int, int> cnt;

int main() { cin.tie(0)->sync_with_stdio(0);
	cin >> n;
	for (int i = 1, x; i <= n; i++) {
		cin >> x;
		cnt[x]++;
	}
	for (auto x : cnt) {
		if (x.second >= 2) {
			v.push_back(x.first);
		}
	}
	if (size(v) < 2) return cout << 0, 0;
	sort(v.begin(), v.end());
	cout << 1ll * v[size(v) - 2] * v.back();
}