#include <iostream>
#include <algorithm>
#include <cstring>
#include <string>
#include <vector>
#include <queue>
#include <deque>
#include <cmath>

#define AS(a) (sizeof(a)/sizeof(a[0]))
using namespace std;
long long int MOD = 1000000007;
long long int INF = 1000000007;


int main() {
  string s;
  cin >> s;
  int count = 0;
  while(1) {
    bool b = true;
    for(int i = 1; i < s.length(); i++) {
      if(s[i-1] == 'B' && s[i] == 'A') {
        count++;
        s[i-1] = 'A';
        s[i] = 'B';
        b = false;
        break;
      }
    }
    if(b) break;
  }
  cout << count << endl;


}