Problem:
Score : 200 points

Problem Statement

You are given a positive integer X.
Find the largest perfect power that is at most X.
Here, a perfect power is an integer that can be represented as b^p, where b is an integer not less than 1 and p is an integer not less than 2.

Constraints

1 ≤ X ≤ 1000

X is an integer.

Input

Input is given from Standard Input in the following format:

X

Output

Print the largest perfect power that is at most X.

Sample Input 1

10

Sample Output 1

9

There are four perfect powers that are at most 10: 1, 4, 8 and 9.
We should print the largest among them, 9.

Sample Input 2

1

Sample Output 2

1

Sample Input 3

999

Sample Output 3

961

Solution in Rust (1.15.1) (judge verdict: Wrong Answer)

use std::io;

fn read_line()  -> String {
    let mut buf = String::new();
    io::stdin().read_line(&mut buf).expect("Couldn't read standard input");
    buf.trim().to_owned()
}

fn main() {
    let x: i32 = read_line().parse().unwrap();

    for n in (0 .. x).rev() {
        if x <= 1 {
            println!("1");
            return;
        }

        for i in 2 .. {
            let root: f32 = if (n as f32).powf(1.0 / i as f32) >= 2.0 {
                (n as f32).powf(1.0 / i as f32)
            } else {
                break;
            };

            if root - root.floor() == 0.0 {
                println!("{}", n);
                return;
            }
        }
    }
}