using System;
using System.Collections.Generic;
using System.Linq;
using static Common;

public static class Common
{
    public static void Swap<T>(ref T lhs, ref T rhs)
    {
        T temp = lhs;
        lhs = rhs;
        rhs = temp;
    }
}
class Solution
{
    static void Main()
    {
        var s = Console.ReadLine().ToArray();
        var aCount = s.Count(c => c == 'A');
        int count = 0;

        while (!s.Take(aCount).All(c => c == 'A'))
        {
            var target = 0;
            for (; true; target++)
            {
                if (s[target] == 'B' && s[target + 1] == 'A')
                {
                    break;
                }
            }

            Swap(ref s[target], ref s[target + 1]);
            count++;
        }

        Console.WriteLine(count);
    }
}