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 Java (OpenJDK 11.0.6) (judge verdict: Wrong Answer)



import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.TreeMap;

public class Main {

	static Scanner scanner;
	public static void main(String[] args) {
	    scanner = new Scanner(System.in);

	    int N=gi();
        Map<Integer, Integer> map=new TreeMap<>(Collections.reverseOrder());

	    for(int i=0; i<N; i++) {
	        int a=gi();
	        if(map.containsKey(a)) {
	        	map.put(a, map.get(a)+1);
	        } else {
	        	map.put(a, 1);
	        }
	    }
	    
	    int x=0;
	    int y=0;
	    for(Map.Entry<Integer, Integer> e : map.entrySet()) {
	    	int v=e.getValue();
	    	if(v>1) {
	    		if(x==0) {
	    			x=e.getKey();
	    		} else {
	    			y=e.getKey();
	    			break;
	    		}
	    	}
	    }

		System.out.println(x*y);
	}

	// 文字列として入力を取得
	public static String gs() {
		return scanner.next();
	}

	// intとして入力を取得
	public static int gi() {
		return Integer.parseInt(scanner.next());
	}

	// longとして入力を取得
	public static long gl() {
		return Long.parseLong(scanner.next());
	}

	// doubleとして入力を取得
	public static double gd() {
		return Double.parseDouble(scanner.next());
	}
}