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: Wrong Answer)

use std::collections::HashMap;

#[allow(unused_macros)]
macro_rules! get {
    ([$t:ty] ; $n:expr) => {
        (0..$n)
        .map(|_| get!([$t])).collect::<Vec<_>>()
    };
    ($($t:ty),+ ; $n:expr) => {
        (0..$n).map(|_| get!($($t),+)).collect::<Vec<_>>()
    };
    ([$t:ty]) => {
        rl().split_whitespace()
        .map(|x| x.parse().unwrap())
        .collect::<Vec<$t>>()
    };
    ($t:ty) => {
        rl().parse::<$t>().unwrap()
    };
    ($($t:ty),*) => {
        {
            let line =rl();
            let mut iter = line.split_whitespace();
            ($(iter.next().unwrap().parse::<$t>().unwrap()),*)
        }
    };
}
fn rl() -> String {
    let mut line = String::new();
    std::io::stdin().read_line(&mut line).unwrap();
    line.trim().to_string()
}

fn main() {
    let _n = get!(usize);
    let v = get!([usize]);
    let mut map = HashMap::new();
    for i in 0..v.len() {
        let counter = map.entry(v[i]).or_insert(0);
        *counter += 1;
    }
    //eprintln!("{:?}", map);
    let mut e1: usize = 0;
    let mut e2: usize = 0;
    for (key, val) in map.iter() {
        if *val <= 1 {
            continue;
        }
        if *key < e1 && *key < e2 {
            continue;
        }
        if 4 <= *val && (e1 * e2 < *val * *val) {
            if e1 < *key && e2 < *key {
                e1 = *key;
                e2 = *key;
            } else {
                if e2 < e1 {
                    e2 = *key;
                } else {
                    e1 = *key;
                }
            }
        }
        if e1 <= *key || e2 <= *key {
            if e1 <= e2 {
                e1 = *key;
            } else {
                e2 = *key;
            }
        }
        //eprintln!("e1: {}, e2: {}", e1, e2);
    }
    println!("{}", e1 * e2);
}