#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 5;
const int MOD = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  string s;
  cin >> s;
  sort(s.begin(), s.end());
  long long int sz = (int)s.size();
  int i = 0, ct = 0;
  while (s[i] == 'a') {
    ct++;
    i++;
  }
  while (sz && ct * 2 <= sz) {
    char ch = s.back();
    sz--;
    if (ch == 'a') ct--;
  }
  cout << sz;
  return 0;
}
