#include <bits/stdc++.h>
using namespace std;

long long gcd(long long x, long long y)
{
    if (y == 0)
        return x;
    return gcd(y, x % y);
}

long long lcm(long long x, long long y)
{
    if (x == 0 || y == 0)
        return 0;
    return x / gcd(x, y) * y;
}

int main()
{
    string s;
    cin >> s;
    int n = s.size();
    string ss = s;
    sort(ss.begin(), ss.end());

    int cnt = 0;
    while (ss != s)
    {
        for (int i = 0; i < n - 1; i++)
        {
            if (s[i] == 'B' and s[i + 1] == 'A')
            {
                cnt++;
                swap(s[i], s[i + 1]);
            }
        }
        /*
        for (int i = n - 1; i < 0; i--)
        {
            if (s[i] == 'A' and s[i - 1] == 'B')
            {
                cnt++;
                swap(s[i], s[i - 1]);
            }
        }
        */
    }
    cout << cnt << endl;
}