using System;

public class Hello
{
    public static void Main()
    {
        var s = Console.ReadLine().Trim();
        var sL = s.Length;
        var count = 0;
        while (true)
        {
            var a = (sL - s.Replace("BA", "").Length) / 2;
            if (a == 0) break;
            s = s.Replace("BA", "AB");
            count += a;
        }
        Console.WriteLine(count);
    }
}