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 (1.14.1) (judge verdict: Accepted)

package main

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

var sc = bufio.NewScanner(os.Stdin)

func readInt() int {
	sc.Scan()
	iv, _ := strconv.Atoi(sc.Text())
	return iv
}

func init() {
	sc.Split(bufio.ScanWords)
}

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

	type stick struct{ a, c int }
	sts := make([]stick, 0, len(m))
	for a, c := range m {
		sts = append(sts, stick{a, c})
	}
	sort.Slice(sts, func(i, j int) bool { return sts[i].a > sts[j].a })
	t := 0
	for _, st := range sts {
		if t == 0 {
			if st.c >= 4 {
				fmt.Println(st.a * st.a)
				return
			} else if st.c >= 2 {
				t = st.a
			}
		} else {
			if st.c >= 2 {
				fmt.Println(t * st.a)
				return
			}
		}
	}
	fmt.Println(0)
}