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#include <iostream>

using namespace std;
string a;
int ile;
int sum;
int main()
{
    std::ios_base::sync_with_stdio(0);
    cin>>a;
    for(int i=0;i<a.size();i++)
    {
        if(a[i]=='a')ile++;
    }
    if(a.size()%2==0&&ile%2==1)
    {
        cout<<-1;
        return 0;
    }
    int ii=0;
    int jj=a.size()-1;
    int i=ii;
    int j=jj;
    while(1)
    {
        while(i<jj&&a[i]!='b')i++;
        while(j>ii&&a[j]!='b')j--;
        if(i>j)
        {
            cout<<sum;
            return 0;
        }
        if(i==j)
        {
            sum+=abs((int)a.size()/2-i);
            cout<<sum;
            return 0;
        }
        else
        {
            sum+=abs(j-(jj-i));
            i++;
            j--;
        }
    }
    return 0;
}