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

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        var s = ReadLine();
        var dp = Enumerable.Repeat(int.MaxValue, s.Length * 2 + 1).ToArray();
        var offset = s.Length;
        dp[offset] = -1;
        var tmp = 0;
        var ans = 0;
        for (var i = 0; i < s.Length; ++i)
        {
            if (s[i] == 'A') ++tmp;
            else --tmp;
            dp[tmp + offset] = Math.Min(dp[tmp + offset], i);
            ans = Math.Max(ans, i - dp[tmp + offset]);
        }
        WriteLine(ans);
    }
}