use std::io::*;
use std::str::FromStr;
use std::cmp::{min, max};
use std::mem::swap;
use std::collections::HashMap;

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

static DX: &'static [i32] = &[0, 0, 1, -1];
static DY: &'static [i32] = &[1, -1, 0, 0];

fn main() {
    let s: String = read();

    let mut cnt = vec![0 as i32; s.len() + 1];

    let mut t = 0;

    for (i, c) in s.chars().enumerate() {
        if c == 'A' {
            t += 1;
        } else {
            t -= 1;
        }

        cnt[i + 1] = t;
    }

    let mut ans = 0;

    for i in 0..cnt.len() - 1 {
        for j in i + 1..cnt.len() {
            if cnt[i] == cnt[j] {
                ans = max(ans, j - i);
            }
        }
    }

    println!("{}", ans);
}