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

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	int i;
	string s;
	cin>>s;
	int n = s.size();
	int ile1 = 0;
	int ile2 = 0;
	for (i = 0; i < n; i++){
		if (s[i] == 'a') ile1++;
		else ile2++;
	}
	//cout<<ile1<<" "<<ile2<<"\n";
	if (n%2){
		if (ile1%2 && ile2%2){
			cout<<"-1\n";
			return 0;
		}
	}else if (ile1%2 || ile2%2){
		cout<<"-1\n";
		return 0;
	}
	int wy = 0;
	for (i = 0; i < n/2; i++){
		if (s[i] == s[n-i-1]) continue;
		wy++;
		if (s[i+1] == s[n-i-1]){
			swap(s[i],s[i+1]);
		}else{
			swap(s[n-i-1],s[n-i-2]);
		}
	}
	cout<<wy<<"\n";
	return 0;
}