using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace YukiCoder
{
	public class SwapAB
	{
		public static void　Main()
		{
			int SwapCount = 0;

			List<char> SwapString = Console.ReadLine().ToCharArray().ToList<char>();

			int doCount = SwapString.Count;

			while (--doCount != 0)
			{
				for (int i = 0; i < SwapString.Count - 1; i++)
				{

					if (SwapString[i] == 'B' && SwapString[i + 1] == 'A')
					{
						SwapString[i] = 'A';
						SwapString[i + 1] = 'B';
						SwapCount++;
						continue;
					}
				}
			}

			Console.WriteLine(SwapCount);
		}


	}
}