import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int ans = function(s);
        System.out.println(ans);
    }
    
    static int function(String s) {
        int len = s.length();
        int [] num = new int [len];
        if(s.charAt(0) == 'A') {
            num[0] = 1;
        } else {
            num[0] = -1;
        }
        for(int i = 1; i < len; i++) {
            if(s.charAt(i) == 'A') {
                num[i] = num[i-1] + 1;
            } else {
                num[i] = num[i-1] - 1;
            }
        }
        int max = 0;
        for(int i = 0; i < len; i++) {
            for(int j = i; j < len; j++) {
                if(num[j]-num[i] == 0) {
                    max = Math.max(max,j-i);
                }
            }
        }
        return max;
    }
}