using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Competitive.lib
{

    internal class Solution
    {
        public Dictionary<int, int> FST;

        public void Run()
        {
            string S = Console.ReadLine();
            int N = S.Length;

            FST = new Dictionary<int, int>();
            int ret = 0;

            int c = 0;
            FST.Add(0, 0);

            for (int i = 0; i < N; i++)
            {
                if (S[i] == 'A') c++;
                else c--;
                if (FST.ContainsKey(c))
                {
                    ret = Math.Max(ret, i - FST[c]);
                }
                else
                {
                    FST.Add(c, i);
                }
            }

            Console.WriteLine(ret);
        }
    }

    // libs ----------
    

    // common ----------

    internal static class Input
    {
        public static int ReadInt()
        {
            string line = Console.ReadLine();
            return int.Parse(line);
        }

        public static int[] ReadIntArray()
        {
            string line = Console.ReadLine();
            return line.Split(' ').Select(int.Parse).ToArray();            
        }

        public static double[] ReadDoubleArray()
        {
            string line = Console.ReadLine();
            return line.Split(' ').Select(double.Parse).ToArray();
        }
    }
    
    internal class Program
    {
        public static void Main(string[] args)
        {
            var s = new Solution();
            s.Run();
        }
    }
}