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 l = S.length();
		int cnt1 = 0;
		//int cnt2 = 0;
		for(int i = 0; i < l; i++) {
			char c = S.charAt(i);
			if(c == 'A') {
				cnt1 ++;
			}
		}
		//cnt2 = l - cnt1;
		if(l == 1) {
			System.out.println(0);
			System.exit(0);
		}
		int k = l - 1;
		int sum = 0;
		for(int i = cnt1 - 1; i >= 0; i--) {
			char c = S.charAt(i);
			if(c == 'B') {
				for(int j = k; j >= cnt1 - 1; j --) {
					char c2 = S.charAt(j);
					if(c2 == 'A') {
						k = j - 1;
						sum += j - i;
						break;
					}
				}
			}
		}
		System.out.println(sum);

	}
}