#include <bits/stdc++.h>
using namespace std;
struct chash {
  long long operator()(long long x) const { return hash<long long>{}(x); }
};
long long a[100002];
bool prime[100001];
void f2(long long n) {
  for (long long p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (long long i = p * p; i <= n; i += p) prime[i] = false;
    }
  }
}
long long fun(long long n) {
  unordered_map<long long, long long> pri;
  for (long long i = 2; i <= sqrt(n); ++i) {
    while (n % i == 0) {
      ++pri[i];
      n = n / i;
    }
  }
  if (n >= 2) ++pri[n];
  long long ans1 = 1;
  for (auto it : pri) {
    ans1 *= 2 * it.second + 1;
  }
  return ans1;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string s;
  cin >> s;
  long long n = s.length(), i;
  long long a = 0;
  for (i = 0; i < n; i++) {
    if (s[i] == 'a') a++;
  }
  cout << min(n, 2 * a - 1) << "\n";
  return 0;
}
