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#include <iostream>
#include <vector>
using namespace std;

string s;
int S, n;
vector <int> b;

int main()
{
	cin >> s;
	n = s.length();
	for (int i = 0; i < n; i++)
	{
		if (s[i] == 'b')
			b.push_back(i);
	}
	if (n % 2 == 0)
	{
		if (b.size() % 2)
		{
			cout << -1;
			return 0;
		}
	}
	else
		if (b.size() % 2)
			S += abs(n / 2 - b.at(b.size() / 2));
	for (int i = 0; i < b.size() / 2; i++)
	{
		S += abs(b.at(i) - (n - 1 - b.at(b.size() - 1 - i)));
	}
	cout << S;
}