#define _CRT_SECURE_NO_WARNINGS
#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<string>
#include<algorithm>
#include<cmath>
#include<queue>
#include<vector>
#include<stack>
using namespace std;

#define REP(i,n) for(int i=0;(i)<(n);(i)++)
#define ll long long

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

	int len = S.length();

	int sw = 0;
	REP(k, len) {
		REP(i, len - 1) {
			if (S[i] == 'B' && S[i + 1] == 'A') {
				swap(S[i], S[i + 1]);
				sw++;
			}
		}
	}

	
	cout << sw << endl;
	return 0;
}