using System;
using System.Linq;

namespace y
{
    class Program
    {
        static void Main(string[] args)
        {
            var s = Console.ReadLine();

            int a = 0;
            int b = 0;
            int m = 0;
            for (int i = 0; i < s.Length; i++)
            {
                if (s[i] == 'A')
                {
                    a++;
                    b = s.Substring(0, i).Count(x => x == 'B');
                    m = 2 * (int)Math.Min(a, b);
                }

            }

            Console.WriteLine(m);
        }
    }
}