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

int main(){
    ios_base::sync_with_stdio(0);
    string s;
    cin>>s;
    set<int> pos;
    int n = s.size();
    for(int i=0;i<n;i++){
        if(s[i] == 'a') pos.insert(i);
    }

    long long res=0;
    while(pos.size() >= 2){
        int a = *pos.begin();
        pos.erase(pos.begin());
        int b = *prev(pos.end());
        pos.erase(prev(pos.end()));
        b = n-1-b;
        res += abs(a-b);
    }

    if(pos.size() == 1){
        if(n%2){
            int a = *pos.begin();
            res += abs(a-n/2);
        }
        else res = -1;
    }
    cout<<res<<endl;
}