package yukicoder;

import java.util.Arrays;
import java.util.Scanner;

public class No559 {
    public static void main(String[] args) {
        String str = new Scanner(System.in).next();
        String[] splited = str.split("");

        String tmp;
        int cnt = 0, eachCnt = 0;
        do {
            eachCnt = 0;
            for(int i = 0; i < splited.length - 1; i++) {
                if(splited[i].equals("B") && splited[i + 1].equals("A")) {
                    tmp = splited[i + 1];
                    splited[i + 1] = splited[i];
                    splited[i] = tmp;
                    eachCnt++;
                    cnt++;
                }
            }
        } while(eachCnt != 0);
        System.out.println(cnt);
    }
}