#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<int, int> PI;

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  string s;
  cin >> s;
  int a = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == 'A') a++;
  }
  int ans = 0;
  int pos = 0;
  while (true) {
    bool flag = false;
    for (int i = pos; i < s.size(); i++) {
      if (s[i] == 'B' && s[i+1] == 'A') {
        s[i] = 'A'; s[i+1] = 'B';
        ans++;
        break;
      }
    }
    pos = s.find('B') - 1;
    //cout << s << pos << endl;
    if (pos + 1 == a) {
      break;
    }
  }

  cout << ans << endl;

  return 0;
}