import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		String S = scan.next();
		scan.close();
		int len = S.length();
		Map<Integer, Integer> mapL = new HashMap<Integer, Integer>();
		Map<Integer, Integer> mapR = new HashMap<Integer, Integer>();
		int k = 0;
		mapL.put(0, 0);
		for(int i = 0; i < len; i++) {
			char c = S.charAt(i);
			if(c == 'A') {
				k++;
			}else {
				k--;
			}
			if(!mapL.containsKey(k)) {
				mapL.put(k, i + 1);
			}else {
				mapR.put(k, i + 1);
			}
		}
		int max = 0;
		for(int key : mapL.keySet()) {
			if(mapR.containsKey(key)) {
				int t = mapR.get(key) - mapL.get(key);
				max = Math.max(max, t);
			}
		}
		System.out.println(max);
	}
}