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 z = 200010;

    let mut t = z;

    let mut idx = vec![0; z * 2];
    let mut visited = vec![false; z * 2];

    visited[z] = true;

    let mut ans = 0;

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

        if visited[t] {
            ans = max(ans, i + 1 - idx[t]);
        } else {
            idx[t] = i + 1;
            visited[t] = true;
        }
    }

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