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#include<iostream>
#include<string>
#include<vector>
using namespace std;
#define fio cin.tie(0);ios_base::sync_with_stdio(0);
#define vt vector
int main()
{
    fio;
    string text;
    cin>>text;
    uint64_t ans=0;
    vt<int> posb;
    int dl=text.length();
    for(int i=0;i<dl;i++)
    {
        if(text[i]=='b'){posb.push_back(i+1);}
    }
    if(dl%2==0&&posb.size()%2==1){cout<<"-1";return 0;}

    if(dl%2==0)
    {
        for(int i=0;i<posb.size()/2;i++)
        {
            if(posb[i]-1>dl-posb[posb.size()-i-1])
            {
                ///przesuwamy lewe
                ans+=(posb[i]-1)-(dl-posb[posb.size()-i-1]);
            }
            else
            {
                ans+=(dl-posb[posb.size()-i-1])-(posb[i]-1);
            }
        }
    }
    else
    {
        if(posb.size()%2==1)
        {
            if(posb[posb.size()/2]<((dl/2)+1))
            {
                ans+=((dl/2)+1)-posb[posb.size()/2];
            }
            else
            {
                ans+=posb[posb.size()/2]-((dl/2)+1);
            }
        }
        for(int i=0;i<posb.size()/2;i++)
        {
            if(posb[i]-1>dl-posb[posb.size()-i-1])
            {
                ///przesuwamy lewe
                ans+=(posb[i]-1)-(dl-posb[posb.size()-i-1]);
            }
            else
            {
                ans+=(dl-posb[posb.size()-i-1])-(posb[i]-1);
            }
        }
    
    }
    cout<<ans;
    return 0;
}