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

string s;
int a, b;
LL res;

void f(){
	int A=0;
	for(unsigned i =0; i<s.size()/2; i++){
		if(s[i]=='a')A++;
	}
	if(a/2 < A){
		A = A - a/2;
		int i=s.size()/2-1;
		int j=s.size()/2;
		while(A){
			while(s[i] != 'a'){
				i--;
			}
			while(s[j] != 'b'){
				j++;
			}
			A--;
			res+=j-i;
			swap(s[i], s[j]);
		}
	}else{
		A = a/2 - A;
		int i=s.size()/2-1;
		int j=s.size()/2;
		while(A){
			while(s[i] != 'b'){
				i--;
			}
			while(s[j] != 'a'){
				j++;
			}
			A--;
			res+=j-i;
			swap(s[i], s[j]);
		}
	}
	vector<int> ab;
	vector<int> ba;
	for(unsigned i=0, j=s.size()-1; i<s.size()/2; i++, j--){
		if(s[i]=='a' && s[j]=='b')ab.push_back(i);
		if(s[i]=='b' && s[j]=='a')ba.push_back(i);
	}
	for(unsigned i=0; i<ab.size(); i++){
		res+=abs(ab[i] - ba[i]);
	}
}

void f2(){
	if(b%2){
		for(unsigned i=0; i<s.size(); i++){
			s[i] = s[i]=='a' ? 'b' : 'a';
		}
		swap(a, b);
	}
	int A=0;
	for(unsigned i=0; i<s.size()/2; i++){
		if(s[i]=='a')A++;
	}
	if(s[s.size()/2]=='b'){
		if(a/2 < A){
			for(unsigned i=s.size()/2-1; i>=0; i--){
				if(s[i]=='a'){
					res+=abs(LL(s.size()/2) - LL(i));
					swap(s[s.size()/2], s[i]);
					A--;
					break;
				}
			}
		}
		else{
			for(unsigned i=s.size()/2+1; i<s.size(); i++){
				if(s[i]=='a'){
					res+=abs(LL(s.size()/2) - LL(i));
					swap(s[s.size()/2], s[i]);
					break;
				}
			}
		}
	}
	if(a/2 < A){
		A = A - a/2;
		int i=s.size()/2-1;
		int j=s.size()/2+1;
		while(A){
			while(s[i] != 'a'){
				i--;
			}
			while(s[j] != 'b'){
				j++;
			}
			A--;
			res+=j-i;
			swap(s[i], s[j]);
		}
	}else{
		A = a/2 - A;
		int i=s.size()/2-1;
		int j=s.size()/2+1;
		while(A){
			while(s[i] != 'b'){
				i--;
			}
			while(s[j] != 'a'){
				j++;
			}
			A--;
			res+=j-i;
			swap(s[i], s[j]);
		}
	}
	vector<int> ab;
	vector<int> ba;
	for(unsigned i=0, j=s.size()-1; i<s.size()/2; i++, j--){
		if(s[i]=='a' && s[j]=='b')ab.push_back(i);
		if(s[i]=='b' && s[j]=='a')ba.push_back(i);
	}
	for(unsigned i=0; i<ab.size(); i++){
		res+=abs(ab[i] - ba[i]);
	}
}

int main(){
	ios_base::sync_with_stdio(false);
	cin>>s;
	for(unsigned i=0; i<s.size(); i++){
		if(s[i]=='a')a++;
		else b++;
	}
	if(s.size()%2==0 && (a%2 || b%2)){
		cout<<"-1\n";
		return 0;
	}
	
	if(s.size()%2==0){
		f();
		cout<<res<<"\n";
	}
	else{
		f2();
		cout<<res<<"\n";
	}
	
	return 0;
}