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//Dawid Lebryk
#include <iostream>
#include <queue>

using namespace std;
typedef long long ll;
const int MAXN = 200000;

int tab[MAXN + 1];
deque<int> inda, indb;

void add(int a)
{
	for (int i = a; i > 0; i -= (i & -i))
	{
		tab[i]++;
	}
}

int sum(int a)
{
	int s = 0;
	for (int i = a; i <= MAXN; i += (i & -i))
	{
		s += tab[i];
	}
	return s;
}

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	string s;
	cin >> s;
	ll n = s.size();
	for (int i = 1; i <= n; i++)
	{
		if (s[i - 1] == 'a')
		{
			inda.push_back(i);
		}
		else
		{
			indb.push_back(i);
		}
	}
	if (inda.size() % 2 == 1 && indb.size() % 2 == 1)
	{
		cout << -1;
		return 0;
	}
	ll w = 0;
	for (int i = 1; i <= n; i++)
	{
		if (s[i - 1] == 'a')
		{
			if (!inda.empty())
			{
				ll p = n - inda.back() - sum(inda.back());
				add(inda.back());
				if (inda.size() > 1) inda.pop_back();
				else p /= 2;
				inda.pop_front();
				w += p;
			}
		}
		else
		{
			if (!indb.empty())
			{
				ll p = n - indb.back() - sum(indb.back());
				add(indb.back());
				if (indb.size() > 1) indb.pop_back();
				else p /= 2;
				indb.pop_front();
				w += p;
			}
		}
	}
	cout << w;
	return 0;
}