import java.util.Scanner

fun inputListSplit(): MutableList<String>{
	var input = (Scanner(System. `in`).next()).split(Regex(""))
	var list: MutableList<String> = mutableListOf()
	for(i in 0..input.count() - 2) list.add(input[i])
	return list
}

fun swap(list: MutableList<String>): Int{
	var count: Int = 0
	var i: Int = 0
	while(i < list.count() - 1){
		if(list[i] == "B" && list[i + 1] == "A"){
			list[i] = list[i + 1].also {list[i + 1] = list[i]}
			count++
			if(i != 0 && list[i - 1] == "B") i -= 2
		}
		i++
	}
	return count
}

fun main(args: Array<String>){
	var list: MutableList<String> = inputListSplit()
	println( swap(list) )
}