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::io::*;
use std::str::*;

const MOD: i32 = 1000000007;

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

fn main() {
    let n: usize = read().unwrap();
    let mut v = Vec::new();
    for i in 0..n {
        let tmp = read().unwrap();
        v.push(tmp);
    }
    v.sort();

    let mut p = 0;
    let mut i = n - 1;
    for _ in (1..n).rev() {
        //        println!("{}", i);
        if p != 0 {
            if (v[i] == v[i - 1]) {
                println!("{}", p * v[i]);
                return;
            }
        }
        if (v[i] == v[i - 1]) {
            p = v[i];
            if i == 0 {
                break;
            }
            i -= 1;
        }
        if i == 0 {
            break;
        }
        i -= 1;
    }
    println!("{}", 0);
}