package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"strings"
)

func main() {
	maxSize := 2 * int(math.Pow10(5))
	rdr := bufio.NewReaderSize(os.Stdin, maxSize)
	buf := make([]byte, 0, maxSize)
	for {
		l, p, _ := rdr.ReadLine()
		buf = append(buf, l...)
		if !p {
			break
		}
	}
	s := string(buf)

	sl := len(s)
	al := strings.Count(s, "A")
	bl := sl - al
	limit := int(math.Min(float64(al), float64(bl))) * 2
	max := 0
	for i := limit; i >= 2; i -= 2 {
		for j := 0; j < sl-i; j++ {
			if strings.Count(s[j:j+i], "A")*2 == i {
				max = i
				break
			}
		}

		if max > 0 {
			break
		}
	}
	fmt.Println(max)
}