#include <iostream>
#include <string>
using namespace std;
int main(){
	string S;
	cin >> S;
	int N = S.size();
	int ans = 0;
	for (int i = 0; i < N; i++){
		for (int j = 0; j < N - 1; j++){
			if (S[j] == 'B' && S[j + 1] == 'A'){
				ans++;
				swap(S[j], S[j + 1]);
			}
		}
	}
	cout << ans << endl;
}