// 入力関数
fn read<T: std::str::FromStr>() -> T {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    s.trim().parse().ok().unwrap()
}

use std::cmp;
// 以下main
fn main() {
    let mut s: String = read();

    let n = s.len();
    s.retain(|k| k == '1');
    let n_0 = s.len();

    println!("{}", 2 * cmp::min(n_0, n - n_0));
}
