#include <iostream>
#include <string>

using namespace std;

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

    int len = ab.length();
    int bpos;

    for (int i = 0; i < len; i++) {
        if (ab[i] == 'B') {
            bpos = i;
            break;
        }
    }

    int apos;

    for (int i = len - 1; i >= 0; i--) {
        if (ab[i] == 'A') {
            apos = i;
            break;
        }
    }

    cout << apos - bpos << "\n";

    int a;
    cin >> a;
}