#include <bits/stdc++.h>

using namespace std;

bool check(string s) {
    string s2 = s;
    sort(s2.begin(), s2.end());
    return s == s2;
}

int main() {
    string s;
    cin >> s;
    int cnt = 0;
    while (!check(s)) {
        for (int i = 0; i < s.size() - 1; i++) {
            if (s[i] == 'B' && s[i + 1] == 'A') {
                swap(s[i], s[i + 1]);
                cnt++;
                break;
            }
        }
    }
    cout << cnt << endl;
    return 0;
}