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#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(a, b) for(int a = 0; a < (b); ++a)
#define pb push_back
int main() {
  ios_base::sync_with_stdio(0); cin.tie(0);
  string s;
  cin >> s;
  rep(i, 2) {
    vector<int>V;
    rep(j, s.size()) if(s[j]=='a'+i) V.pb(j);
    if(V.size()%2==1) continue;
    ll ans=0;
    rep(j, V.size()/2) {
      ll x=s.size()-V[V.size()-j-1]-1;
      ans+=abs((long long)V[j]-x);
    }
    cout << ans << '\n';
    return 0;
  }
  cout << -1 << '\n';
}