import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String s = sc.next();
    int ans = 0;
    int p = 0;
    HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
    map.put(0, 0);
    for(int i = 0; i < s.length(); i++) {
      if(s.charAt(i) == 'A') p++;
      if(s.charAt(i) == 'B') p--;
      if(map.containsKey(p)) {
        ans = Math.max(ans, i + 1 - map.get(p));
      } else {
        map.put(p, i + 1);
      }
    }
    System.out.println(ans);
  }
}