using System.Collections.Generic;
using System;

public class Hello
{
    public static void Main()
    {
        var s = Console.ReadLine().Trim();
        var sL = s.Length;
        var a = new int[sL];
        for (int i = 0; i < sL; i++)
            a[i] = s[i] == 'A' ? 1 : -1;
        var d = new Dictionary<int, int>();
        var ans = 0;
        d[a[0]] = 0;
        for (int i = 1; i < sL; i++)
        {
            a[i] +=  a[i - 1];
            if (d.ContainsKey(a[i]))
                ans = Math.Max(ans, i - d[a[i]]);
            else d[a[i]] = i;
        }
        Console.WriteLine(ans);
    }
}