using System;
using System.Collections.Generic;

namespace No672
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var S = Console.ReadLine();
            var dic = new Dictionary<int, int> {[0] = 0};

            var count = 0;
            var ans = 0;
            for (var i = 1; i <= S.Length; i++)
            {
                count += (S[i - 1] - 'A') * 2 - 1;
                if (dic.TryGetValue(count, out var dist)) ans = Math.Max(ans, i - dist);
                else dic.Add(count, i);
            }

            Console.WriteLine(ans);
        }
    }
}