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 Rust (1.15.1) (judge verdict: Accepted)

use std::io::*;
use std::str::*;
use std::collections::*;

fn read<T: FromStr>() -> Option<T> {
    let stdin = stdin();
    let s = stdin.bytes().map(|c| c.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect::<String>();
    if s.is_empty() {
        None
    } else {
        s.parse::<T>().ok()
    }
}

fn main(){
    while let Some(n) = read() {
        let mut mp = BTreeMap::<u64, usize>::new();
        let mut v: Vec<u64> = (0..n).map(|_| read().unwrap()).collect();
        v.sort();
        for &x in &v {
            *mp.entry(x).or_insert(0) += 1;
        }
        let mut e = Vec::<u64>::new();
        v.dedup();
        for &x in &v {
            for _ in 0..mp[&x]/2 {
                e.push(x);
            }
        }
        e.sort();
        let ans = if e.len() < 2 {
            0
        } else {
            let l = e.len();
            e[l - 1] * e[l - 2]
        };
        println!("{}", ans);
    }
}