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#include <bits/stdc++.h>
using namespace std;
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	string x;
	cin>>x;
	vector <long long> v;
	for(long long i=0; i<x.size(); i++){
		if(x[i]=='a'){
			v.push_back(i);
		}
	}
	if(x.size()%2==0&&v.size()%2==1){
		cout<<"-1";
		return 0;
	}
	long long wynik=0;
	for(long long i=0; i<v.size()/2; i++){
		wynik+=max(v[i]-(long long)x.size()+1+v[v.size()-1-i], (long long)x.size()-1-v[v.size()-1-i]-v[i]);
	}
	if(v.size()%2==1){
		wynik+=max((long long)x.size()/2-v[v.size()/2], v[v.size()/2]-(long long)x.size()/2);
	}
	cout<<wynik;
}