using `Result<T, Box<Error>>` to allow `?` operator

error.rs

use std::error::Error;

fn run() -> Result<(), Box<Error>> {
    let stdin = std::io::stdin();
    let mut buffer = String::new();
    stdin.read_line(&mut buffer)?;
    let n: u32 = buffer.parse()?;
    println!("{}", n);
    Ok(())
}

fn main() {
    run().unwrap();
}