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

int main() {
	ios_base::sync_with_stdio(0);
   	cin.tie(0);
	string a;
	int n, x=0, y=0, s=0;
	cin >> a;
	n=a.length();
	for (int i = 0; i < n/2; i++){
		if(a[i]=='a') x++;
		else y++;
		if(a[n-1-i]=='a') x++;
		else y++;
		if(a[i]!=a[n-1-i]) s++;
	}
	if(n%2==1&&a[n/2+1]=='a') x++;
	if(n%2==1&&a[n/2+1]=='b') y++;
	if(n%2==1&&((a[n/2+1]=='b'&&x%2==1)||(a[n/2+1]=='a'&&y%2==1))) s++;
	s/=2;
	if(y%2==1&&x%2==1) cout << -1;
	else cout << s;
}