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


fn get_one<T : FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    return s.trim().parse().ok().unwrap();
}

fn get_vec<T : FromStr>() -> Vec<T> {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    return s.trim().split_whitespace().map(|e| e.parse().ok().unwrap()).collect();
}

fn main() {
    let n : usize = get_one();
    let v : Vec<i64> = get_vec();

    let mut map = HashMap::new();
    for x in v {
        *map.entry(x).or_insert(0) += 1;
    }
    let mut fst : i64 = 0;
    let mut snd : i64 = 0;
    for (k,v) in map.iter() {
        //println!("{} {}", k, v);
        if v >= &4 {
            if fst < *k {
                snd = *k;
                fst = *k;
            }
            else if snd < *k {
                snd = *k;
            }
        }
        else if v >= &2{
            if fst < *k {
                snd = fst;
                fst = *k;
            }
            else if snd < *k {
                snd = *k;
            }
        }
    }
    println!("{}", fst * snd);
}