#include<iostream>
#include<string>

using namespace std;

int main() {
	string s;
	cin >> s;

	int cnt = 0;
	int len = s.size();
	for (int x = 0; x < len - 1; x++) {
		for (int y = x; y < len - 1; y++) {
			if (s[y] != 'B' || s[y + 1] != 'A') {
				continue;
			}

			char tmp = s[y];
			s[y] = s[y + 1];
			s[y + 1] = tmp;
			cnt++;
		}
	}

	cout << cnt << endl;

    return 0;
}