use std::io::*;

fn main() {
    let mut s: String = String::new();
    std::io::stdin().read_to_string(&mut s).ok();
    let s: Vec<char> = s.trim().chars().collect();
    let n = s.len();

    let mut sum: Vec<i32> = vec![0; 30];
    let mut pos: std::collections::HashMap<i32, Vec<i32>> = std::collections::HashMap::new();
    for i in 0..n {
        if s[i] == 'A' {
            sum[i + 1] += 1;
        } else {
            sum[i + 1] -= 1;
        }
    }
    for i in 0..n {
        sum[i + 1] += sum[i];
    }
    for i in 0..n + 1 {
        let p = pos.entry(sum[i]).or_insert([].to_vec());
        (*p).push(i as i32);
    }

    let mut ans = 0;
    for (_i, v) in pos.iter() {
        for j in 0..v.len() - 1 {
            ans = std::cmp::max(ans, v[j + 1] - v[j]);
        }
    }
    println!("{}", ans);
}