import java.io.*;
import java.util.*;
import java.util.function.*;
import java.util.stream.*;

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