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 (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
)

func main() {
	var N int
	fmt.Scan(&N)
	set := make(map[int]int, 0)
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	for i := 0; i < N; i++ {
		sc.Scan()
		t, _ := strconv.Atoi(sc.Text())
		set[t]++
	}
	ks := make([]int, 0)
	for k := range set {
		ks = append(ks, k)
	}
	sort.Sort(sort.Reverse(sort.IntSlice(ks)))
	m1, m2 := 0, 0
	for i := range ks {
		v := set[ks[i]]
		if v >= 2 {
			if v > m1 {
				m1 = ks[i]
			} else if v > m2 {
				m2 = ks[i]
			}
		}
	}
	ans := 0
	if set[m1] >= 4 {
		ans = m1 * m1
	} else {
		ans = m1 * m2
	}
	fmt.Println(ans)
}