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#include <bits/stdc++.h>

using namespace std;

int main(){

    string s, s1; 
    cin >> s; 
    s1 = s;
    int a = 0, b = 0;

    for(int i = 0; i < s.length(); i++){
        if(s[i] == 'a')
            a++;
        else
            b++;
    }

    if(b%2 == 1 && a % 2 == 1){
        cout << "-1";
        return 0;
    }

    int l = 0, r = s.length() - 1, ans = 0;

    while(l < r){
        if(s[l] == s[r]){
            l++;
            r--;
        }
        else{
            int left = l+1;
            int right = r-1;
            while(left <= r){
                if(s[left] == s[r]){
                    break;
                }
                left++;
            }
            while(right >= l){
                if(s1[right] == s1[l]){
                    break;
                }
                right--;
            }
            if(left - l < r - right){
                ans += left - l;
                s[left] = s[l];
                s[l] = s[r];
            }
            else{
                ans += r - right;
                s[right] = s[r];
                s[r] = s[l];
            }
            l++;
            r--;
        }
    }

    cout << ans;
    return 0;
}


// baaabba -> baaabab
//            0123456 
// abaaabbaababa
// 0123456