using System;
using System.Collections.Generic;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("BA");
            //1
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("B");
            //0
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("ABAABA");
            //4
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();
        string S = InputList[0];

        int Answer = 0;
        while (true) {
            int wkInd = S.IndexOf("BA");
            if (wkInd == -1) break;

            S = S.Remove(wkInd, 2);
            S = S.Insert(wkInd, "AB");
            Answer++;
        }
        Console.WriteLine(Answer);
    }
}