import java.io.*;
import java.math.BigInteger;
import java.util.Arrays;

public class Main implements Runnable {
    static ContestScanner in;
    static Writer out;
    public static void main(String[] args) {
        new Thread(null, new Main(), "", 16 * 1024 * 1024).start();
    }

    public void run() {
        Main main = new Main();
        try {
            in = new ContestScanner();
            out = new Writer();
            main.solve();
            out.close();
            in.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    void solve() throws IOException {
        char[] s = in.nextToken().toCharArray();
        int n = s.length;
        int[] height = new int[n + 1];
        final int base = 200000;
        height[0] = base;
        int[] table = new int[base * 2 + 1];
        Arrays.fill(table, -1);
        table[base] = 0;
        for (int i = 0; i < n; i++) {
            height[i + 1] = height[i] + (s[i] == 'A' ? 1 : -1);
            if (table[height[i + 1]] == -1) table[height[i + 1]] = i + 1;
        }
        int max = 0;
        for (int i = n; i >= 0; i--) {
            max = Math.max(max, i - table[height[i]]);
        }
        System.out.println(max);
    }
}

class Writer extends PrintWriter{
    public Writer(String filename)throws IOException
    {super(new BufferedWriter(new FileWriter(filename)));}
    public Writer()throws IOException {super(System.out);}
}
class ContestScanner implements Closeable {
    private BufferedReader in;private int c=-2;
    public ContestScanner()throws IOException
    {in=new BufferedReader(new InputStreamReader(System.in));}
    public ContestScanner(String filename)throws IOException
    {in=new BufferedReader(new InputStreamReader(new FileInputStream(filename)));}
    public String nextToken()throws IOException {
        StringBuilder sb=new StringBuilder();
        while((c=in.read())!=-1&&Character.isWhitespace(c));
        while(c!=-1&&!Character.isWhitespace(c)){sb.append((char)c);c=in.read();}
        return sb.toString();
    }
    public String readLine()throws IOException{
        StringBuilder sb=new StringBuilder();if(c==-2)c=in.read();
        while(c!=-1&&c!='\n'&&c!='\r'){sb.append((char)c);c=in.read();}
        return sb.toString();
    }
    public long nextLong()throws IOException,NumberFormatException
    {return Long.parseLong(nextToken());}
    public int nextInt()throws NumberFormatException,IOException
    {return(int)nextLong();}
    public double nextDouble()throws NumberFormatException,IOException
    {return Double.parseDouble(nextToken());}
    public void close() throws IOException {in.close();}
}