#define _USE_MATH_DEFINES // for C++  
#include "bits/stdc++.h"
#define LL long long
#define NUM 1000000007

using namespace std;

int main()
{
    int count = 0;
    string s;
    cin >> s;
    if (s.length() == 1 && (s.front() == 'A' || s.front() == 'B'))
    {
        cout << 0 << endl;
        return 0;
    }
    while (true)
    {
        auto result = s.rfind("BA");
        if (result == string::npos)
        {
            cout << count << endl;
            //cout << s << endl;
            return 0;
        }
        iter_swap(s.begin() + result, s.begin() + result + 1);
        count++;
    }
}