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

#define DEBUG(x) cout << #x << ": " << x << endl;
#define DEBUG_VEC(v)                                      \
  cout << #v << ":";                                      \
  for (int i = 0; i < v.size(); i++) cout << " " << v[i]; \
  cout << endl

typedef long long ll;
#define vi vector<int>
#define vl vector<ll>
#define vii vector<vector<int> >
#define vll vector<vector<ll> >
#define vs vector<string>
#define pii pair<int, int>
#define pis pair<int, string>
#define psi pair<string, int>
const int inf = 1000000001;
const ll INF = 1e16;
#define MOD 1000000007
#define mod 1000000009
#define pi 3.14159265358979323846
#define Sp(p) cout << setprecision(15) << fixed << p << endl;
int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};

int main() {
  string s;
  cin >> s;
  int i;
  int cnt = 0;
  bool flag = true;
  while (flag) {
    flag = false;
    for (i = 0; i < s.size() - 1; i++) {
      if (s[i] == 'B' && s[i + 1] == 'A') {
        swap(s[i], s[i + 1]);
        cnt++;
        flag = true;
      }
    }
  }
  cout << cnt << endl;
}