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 (1.42.0) (judge verdict: Accepted)

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

//https://qiita.com/tubo28/items/e6076e9040da57368845
fn read<T: FromStr>() -> T {
    let stdin = stdin();
    let stdin = stdin.lock();
    let token: String = stdin
        .bytes()
        .map(|c| c.expect("failed to read char") as char)
        .skip_while(|c| c.is_whitespace())
        .take_while(|c| !c.is_whitespace())
        .collect();
    token.parse().ok().expect("failed to parse token")
}

fn main() {
    let n: usize = read();
    let mut map = HashMap::new();
    for _ in 0..n{
        let count = map.entry(read::<u64>()).or_insert(0);
        *count += 1;
    }

    let mut counts:Vec<(u64,u64)> = Vec::new();
    for (k,v) in map.iter(){
        counts.push((*k,*v));
    }
    counts.sort_by(|a,b| b.0.partial_cmp(&a.0).unwrap());

    let mut ans: Vec<u64> = Vec::new();
    for v in counts.iter(){
        if v.1 >= 4{
            ans.push(v.0);
            ans.push(v.0);
            break;
        }else if 2 <= v.1 && v.1 < 4{
            ans.push(v.0);
            if ans.len() >= 2{
                break;
            }
        }
    }

    if ans.len() >= 2 {
        println!("{}",ans[0]*ans[1]);
    }else{
        println!("0");
    }

}