inp=input()
a_num=inp.count('a')
if a_num<= len(inp)/2:
print(a_num+a_num-1)
else:
print(len(inp))#include <bits/stdc++.h>
#define int long long
using namespace std;
int32_t main() {
// your code goes here
string s;
cin>>s;
int n=s.length();
int cnta=0;
for(int i=0;i<n;i++){
if(s[i]-'a'==0){
cnta++;
}
}
if(cnta>(n/2)){
cout<<n<<'\n';
}
else cout<<2*cnta-1<<'\n';
return 0;
}
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