#include<iostream>
#include<string>

using namespace std;

int main() {
	string str;
	cin >> str;
	if (str.size() == 1) {
		cout << 0;
		return 0;
	}

	int now = 0;
	int count = 0;
	while(now!=str.size()-1){
		if (str[now] == 'A') {
			now++;
			continue;
		}
		else if (str[now] == 'B') {
			if (str[now + 1] == 'A') {
				char tmp = str[now];
				str[now] = str[now + 1];
				str[now + 1] = tmp;
				now = 0;
				count++;
				continue;
			}
			now++;
		}
	}

	cout << count;

	return 0;
}