fn main(){
    let s = getline();
    let s = s.trim();
    let length:isize = s.len() as isize;
    let c = s.chars().collect::<Vec<char>>();
    let mut ans:i32 = -1;
    // println!("{:?}", s.char_at());
    for i in 0..length {
        ans = check_char(&c, length, length-i);
        if ans != 0 {
            break;
        }
    }
    println!("{}", ans);    
}
fn getline() -> String{
	let mut __ret=String::new();
	std::io::stdin().read_line(&mut __ret).ok();
	return __ret;
}
fn check_char(c: &Vec<char>, length:isize, check_len: isize) -> i32{
    let mut count_a:i32 = 0;
    let mut count_b:i32 = 0;
    let mut i:isize = 0;
    //count A
    //count B
    while i + check_len <= length {
        count_a = 0;
        count_b = 0;
        for j in i..(check_len+i) {
            if c[j as usize] == 'A' {count_a+=1;} 
            if c[j as usize] == 'B' {count_b+=1;} 
        }
        // println!("{}..{} countA={},countB={}", i, check_len+i, count_a, count_b);
        if count_a == count_b {break;}
        i+=1;
    }
    // println!("{:?}", c);
    if count_a == count_b && count_a !=0 {return count_a+count_b}
    return 0;
}