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

int main() {
  string s;
  cin >> s;
  int acnt = 0;
  int n = s.size();
  for (int i = 0; i < n; ++i) {
    if (s[i] == 'A')acnt++;
  }

  vector<int> b;
  vector<int> a;
  for (int i = 0; i < n; ++i) {
    if (s[i] == 'B' && i < acnt) b.push_back(i);
    if (s[i] == 'A' && i >= acnt) a.push_back(i);
  }

  int ans = 0;
  int m = a.size();
  for (int i = 0; i < m; ++i) {
    ans += a[i] - b[i];
  }
  cout << ans << '\n';
  return 0;
}