import java.util.*;

public class Main {
 	public static void main (String[] args) {
    	Scanner sc = new Scanner(System.in);
    	char[] arr = sc.next().toCharArray();
    	int current = 0;
    	int max = 0;
    	HashMap<Integer, Integer> places = new HashMap<>();
    	places.put(0, -1);
    	for (int i = 0; i < arr.length; i++) {
    	    if (arr[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);
	}
}