using System;
using System.Collections.Generic;
using System.ComponentModel;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            String s = Console.ReadLine();
            Int32 n = s.Length;
            var mp = new Dictionary<Int32,Int32>();
            Int32 p = 0;
            mp[0] = -1;
            Int32 ans = 0;
            for (Int32 i = 0; i < n; i++)
            {
                if (s[i] == 'A') p++;
                else p--;
                if (mp.ContainsKey(p)) ans = Math.Max(ans, i - mp[p]);
                else mp[p] = i;
            }
            Console.WriteLine(ans);
        }
    }
}