import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        char[] values = sc.next().toCharArray();
        HashMap<Integer, Integer> places = new HashMap<>();
        places.put(0, -1);
        int current = 0;
        int max = 0;
        for (int i = 0; i < values.length; i++) {
            if (values[i] == 'A') {
                current++;
            } else {
                current--;
            }
            if (places.containsKey(current)) {
                max = Math.max(max, i - places.get(current));
            } else {
                places.put(current, i);
            }
        }
        System.out.println(max);
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}