fn func(input_str: &str) -> u32 {
    let (r, b) = input_str.chars().fold((0_u32, 0), |(r, b), x| {
        if x == '0' {
            (r + 1, b)
        } else {
            (r, b + 1)
        } });
    std::cmp::min(r, b) * 2
}

use std::io::Read;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();
    println!("{}", func(&buf[..]));
}