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

class Program
{
    static void Main()
    {
        var S = ReadLine();
        if (!S.Contains('A') || !S.Contains('B')) { WriteLine(0); return; }
        var s = new int[S.Length + 1];
        var dic = new Dictionary<int, int>();
        for (int i = 1; i < s.Length; i++)
        {
            s[i] = s[i - 1] + (S[i - 1] == 'A' ? 1 : -1);
            dic[s[i]] = i;
        }
        var ans = 0;
        for (int i = 0; i < s.Length; i++)
        {
            if (dic.ContainsKey(s[i]))
            {
                ans = Math.Max(ans, dic[s[i]] - i);
            }
        }
        WriteLine(ans);
    }
}