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//
//  main.cpp
//  Palindrom
//
//  Created by Mikołaj Jędrzejewski on 12/8/22.
//

#include <iostream>
#include <vector>

using namespace std;

// i is an index in the first half of the vector a
inline bool right_place(vector<int> &a, int i) {
    return a[i] == a[a.size()-1-i];
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    
    //cout<<"Hello my friend!\n";
    vector<int> x;
    int as = 0, bs = 0;
    char c;
    while(cin.get(c))
        if (c == '\n')
            break;
        else
            x.push_back(c == 'a');
    
    for (int i=0; i<x.size(); ++i)
        if (x[i] == 1)
            as++;
        else
            bs++;
    
    if (as%2 == 1 and bs%2 == 1) {
        cout<<-1;
        return 0;
    }
    
    int ans = 0;
    int oks = 0;
    int a1 = 0;         // a's in the first half but on wrong places
    int b1 = 0;         // b's in the first half but on wrong places
        
    for (int i=0; i<x.size()/2; ++i) {
        if (right_place(x, i)) ++oks;
        else if (x[i] == 1) ++a1;
        else                ++b1;
    }
    
    //if (as%2 == 0 or bs%2 == 0)
    //ans=a1+b1-min(a1, b1);
    ans = min(a1, b1) + (max(a1, b1) - min(a1, b1))/2;
    if ((as%2 == 1 and x[x.size()/2] == 0) or (bs%2 == 1 and x[x.size()/2] == 1)) ans++;
    cout<<ans;return 0;
    /*if (as%2 == 1) {
        if (x[ x.size()/2 +1 ] == 1)
            // bab
            ans=a1+b1-min(a1, b1);
        else {
            // bba, the b on the left is actually on the right place
            ans=a1+(b1-1)-min(a1, b1-1) + 1;
        }
    }
    if (bs%2 == 1) {
        if (x[ x.size()/2 +1 ] == 1)
            // bab
            ans=a1+b1-min(a1, b1);
        else {
            // aab, the b on the left is actually on the right place
            ans=(a1-1)+b1-min(a1-1, b1) + 1;
        }
    }
    
    cout<<ans;
    
    return 0;*/
}