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// Author: Kajetan Ramsza
#include "bits/stdc++.h"

using namespace std;

template<typename F, typename S> ostream& operator<<(ostream& os, const pair<F, S> &p) { return os<<"("<<p.first<<", "<<p.second<<")"; }
template<typename T> ostream &operator<<(ostream & os, const vector<T> &v) { os << "{"; typename vector< T > :: const_iterator it;
    for( it = v.begin(); it != v.end(); it++ ) { if( it != v.begin() ) os << ", "; os << *it; } return os << "}"; }

void dbg_out() { cerr<<'\n'; }
template<typename Head, typename... Tail> void dbg_out(Head H, Tail... T) { cerr<<' '<<H; dbg_out(T...); }

#ifdef DEBUG
#define dbg(...) cerr<<"(" << #__VA_ARGS__ <<"):", dbg_out(__VA_ARGS__)
#else
#define dbg(...) 
#endif

typedef long long ll;

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	string s;
	cin>>s;
	int n = s.size();
	int a_num = 0, b_num = 0;
	for(char c : s) {
		if(c == 'a') a_num++;
		else b_num++;
	}
	if(a_num % 2 == 1 && b_num % 2 == 1) {
		cout<<"-1\n";
		return 0;
	}
	if(b_num % 2 == 1) {
		swap(a_num, b_num);
		for(int i=0;i<n;i++) {
			if(s[i] == 'a') s[i] = 'b';
			else s[i] = 'a';
		}
	}
	vector<int> left(b_num);
	vector<int> right(b_num);
	int ind = 0;
	for(int i=0;i<n;i++) {
		if(s[i] == 'b') {
			left[ind++] = i;
			right[b_num-ind] = n-i-1;
		}
	}
	dbg(a_num, b_num);
	dbg(left);
	dbg(right);
	int result = 0;
	for(int i=0;i<b_num/2;i++) {
		result += abs(left[i] - right[i]);
	}
	cout<<result<<'\n';
}