import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		String s = sc.next();
		char[] x = s.toCharArray();
		int n = s.length();
		int ans = 0;
		for(int i = 0 ; i < n ; i++) {
			for(int j = i ; j < n ; j++) {
				char work;
				if(s.charAt(i) == 'B' && s.charAt(j) == 'A') {
					work = s.charAt(i);
					x[i] = s.charAt(j);
					x[j] = work;
					ans++;
				}
			}
		}
		 System.out.println(ans);
 	}
}