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)

// This code is generated by [cargo-atcoder](https://github.com/tanakh/cargo-atcoder)
// Original source code:
/*
#![allow(non_snake_case)]
#![allow(unused_imports)]
 
use proconio::{input, fastout};
use proconio::marker::*;
use whiteread::parse_line;
use std::collections::*;

use itertools::Itertools;

#[fastout]
fn main() {
    input!{
        n: usize,
        a_vec: [usize; n],
    }

    let mut bmap: BTreeMap<usize, usize> = BTreeMap::new();
    
    for a in a_vec.iter() {
        *bmap.entry(*a).or_insert(0) += 1;
    }

    let ans_vec: Vec<(&usize, &usize)> = bmap.iter().filter(|v| v.1 >= &2).collect();
    let mut ans = 0;
    if ans_vec.len() == 0 || (ans_vec.len() == 1 && ans_vec[0].1 < &4) {
        ans = 0;  
    } else if ans_vec[ans_vec.len() - 1].1 >= &4 {
        ans = ans_vec[ans_vec.len() - 1].0 * ans_vec[ans_vec.len() - 1].0
    } else {
        ans = ans_vec[ans_vec.len() - 1].0 * ans_vec[ans_vec.len() - 2].0
    }

    println!("{}", ans);
}
*/

fn main() {
    let exe = "/tmp/binA1457C8B";
    std::io::Write::write_all(&mut std::fs::File::create(exe).unwrap(), &decode(BIN)).unwrap();
    std::fs::set_permissions(exe, std::os::unix::fs::PermissionsExt::from_mode(0o755)).unwrap();
    std::process::exit(std::process::Command::new(exe).status().unwrap().code().unwrap())
}

fn decode(v: &str) -> Vec<u8> {
    let mut ret = vec![];
    let mut buf = 0;
    let mut tbl = vec![64; 256];
    for i in 0..64 { tbl[TBL[i] as usize] = i as u8; }
    for (i, c) in v.bytes().filter_map(|c| { let c = tbl[c as usize]; if c < 64 { Some(c) } else { None } }).enumerate() {
        match i % 4 {
            0 => buf = c << 2,
            1 => { ret.push(buf | c >> 4); buf = c << 4; }
            2 => { ret.push(buf | c >> 2); buf = c << 6; }
            3 => ret.push(buf | c),
            _ => unreachable!(),
        }
    }
    ret
}

const TBL: &'static [u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
const BIN: &'static str = "<base64-encoded ELF executable removed>";