package main

import (
	"fmt"
	"strings"
)

func main() {
	var S string
	fmt.Scan(&S)
	s := strings.Split(S, "")
	cnt := 0
	for {
		swap := false
		for i := 0; i < len(S)-1; i++ {
			j := i + 1
			if s[i] == "B" && s[j] == "A" {
				s[i], s[j] = s[j], s[i]
				cnt++
				swap = true
			}
		}
		if !swap {
			break
		}
	}
	fmt.Println(cnt)
}