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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"fmt"
	"sort"
)

func main() {
	var n int
	fmt.Scan(&n)
	m := make(map[int]int, n)
	var a int
	for i := 0; i < n; i++ {
		fmt.Scan(&a)
		m[a]++
	}

	b := []int{}
	for k, v := range m {
		if v >= 2 {
			b = append(b, k)
			v -= 2
		}
		if v >= 2 {
			b = append(b, k)
		}
	}
	sort.Ints(b)

	if len(b) >= 2 {
		fmt.Println(b[len(b)-1] * b[len(b)-2])
	} else {
		fmt.Println(0)
	}
}