import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;

public class Main {
	
	public static final long MOD = 1000000007;
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final char[] chs = sc.next().toCharArray();
		int[] score = new int[chs.length];
		for(int i = 0; i < chs.length; i++){
			if(chs[i] == 'A'){
				score[i] = 1;
			}else{
				score[i] = -1;
			}
		}
		
		for(int i = 1; i < score.length; i++){
			score[i] += score[i - 1];
		}
		
		final int min = Math.min(0, Arrays.stream(score).min().getAsInt());
		final int max = Math.max(0, Arrays.stream(score).max().getAsInt());
		final int size = max - min + 1;
		int[] prev = new int[size];
		Arrays.fill(prev, score.length);
		prev[0 - min] = -1;
		
		int answer = 0;
		for(int i = 0; i < score.length; i++){
			final int s = score[i] - min;
			if(prev[s] < score.length){
				answer = Math.max(answer, i - prev[s]);
			}else{
				prev[s] = i;
			}
		}
		
		System.out.println(answer);
	}
}