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"
	"strconv"
)

func solve() {
	n := getInt()
	m := make(map[int]int)
	for i := 0; i < n; i++ {
		a := getInt()
		m[a]++
	}

	first, second := 0, 0
	for a, num := range m {
		cnt := 0
		for num >= 2 {
			if a > first {
				second = first
				first = a
			} else if a > second {
				second = a
			}

			num -= 2
			cnt++
			if cnt == 2 {
				break
			}
		}
	}

	ans := first * second
	fmt.Fprintln(wr, ans)
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

// -----------------------------------------

const (
	inf = 1 << 60
	// mod = 1000000007
)

var (
	sc = bufio.NewScanner(os.Stdin)
	wr = bufio.NewWriter(os.Stdout)
)

func main() {
	// buf := 200001
	// sc.Buffer(make([]byte, buf), buf)
	sc.Split(bufio.ScanWords)
	solve()
	wr.Flush()
}

func getInt() int {
	sc.Scan()
	i, err := strconv.Atoi(sc.Text())
	if err != nil {
		panic(err)
	}
	return i
}

func getIntSlice(n int) []int {
	is := make([]int, n)
	for i := range is {
		is[i] = getInt()
	}
	return is
}

func getString() string {
	sc.Scan()
	s := sc.Text()
	return s
}

func getRunes() []rune {
	return []rune(getString())
}