import java.util.*;

public class Main {
	public static void main (String[] args) {
		Scanner sc = new Scanner(System.in);
		char[] arr = sc.next().toCharArray();
		int[] sums = new int[arr.length + 1];
		HashMap<Integer, TreeSet<Integer>> map = new HashMap<>();
		TreeSet<Integer> first = new TreeSet<>();
		first.add(0);
		map.put(0, first);
		for (int i = 1; i <= arr.length; i++) {
		    sums[i] = sums[i - 1];
		    if (arr[i - 1] == 'A') {
		        sums[i]++;
		    } else {
		        sums[i]--;
		    }
		    if (map.containsKey(sums[i])) {
		        map.get(sums[i]).add(i);
		    } else {
		        TreeSet<Integer> set = new TreeSet<>();
		        set.add(i);
		        map.put(sums[i], set);
		    }
		}
		int max = 0;
		for (TreeSet<Integer> set : map.values()) {
		    max = Math.max(max, set.last() - set.first());
		}
		System.out.println(max);
   }
}