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::collections::HashMap;
use std::io::prelude::*;
use std::io::stdin;
use std::str::FromStr;

fn main() {
    let n: usize = read();
    let mut map = HashMap::with_capacity(n);

    for _ in 0..n {
        let a_i: u64 = read();
        *map.entry(a_i).or_insert(0) += 1;
    }
    let mut v1: Vec<u64> = map
        .iter()
        .filter(|&(_, v)| *v >= 4)
        .map(|(k, _)| *k)
        .collect();

    v1.sort();
    let rec1 = if v1.len() >= 1 {
        v1[v1.len() - 1].pow(2)
    } else {
        0
    };

    let mut v: Vec<u64> = map
        .iter()
        .filter(|&(_, v)| *v >= 2)
        .map(|(k, _)| *k)
        .collect();
    v.sort();

    let rec2 = if v.len() >= 2 {
        v[v.len() - 1] * v[v.len() - 2]
    } else {
        0
    };
    println!("{}", std::cmp::max(rec1, rec2));
}

fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let token: String = stdin
        .lock()
        .bytes()
        .map(|b| b.unwrap() as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().unwrap()
}