fn main() {
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).ok();
    let s: &str = s.trim();
    let mut result: usize = 0;
    let mut b_count: usize = 0;

    s.chars().for_each(|c| {
        if c == 'A' {
            result += b_count;
        } else {
            b_count += 1;
        }
    });

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