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#include <iostream>
#include <string>
#include <vector>
#include <algorithm>


using namespace std;


using lli = long long int;

int iCntA = 0;
int iCntB = 0;

vector<int> vi;


static char GetChar2LookAt()
{
	if (iCntA % 2 == 1)
	{
		return 'b';
	}

	if (iCntB % 2 == 1)
	{
		return 'a';
	}

	if (iCntA <= iCntB)
	{
		return 'a';
	}

	return 'b';
}


int main()
{
	string s;

	cin >> s;

	for (int i = 0; i < s.size(); ++i)
	{
		if (s[i] == 'a')
		{
			iCntA++;
		}
		else
		{
			iCntB++;
		}
	}

	if (iCntA % 2 == 1 && iCntB % 2 == 1)
	{
		cout << -1;
		return 0;
	}

	if (iCntA == 0 || iCntB == 0)
	{
		cout << 0;
		return 0;
	}

	char c = GetChar2LookAt();

	for (int i = 0; i < s.size(); ++i)
	{
		if (s[i] == c)
		{
			vi.push_back(i);
		}
	}

	int iStart = 0;
	int iEnd = vi.size() - 1;
	lli iResult = 0;

	while (iStart < iEnd)
	{
		int iFromLeft = vi[iStart];

		int iFromRight = s.size() - 1 - vi[iEnd];

		iResult += abs(iFromLeft - iFromRight);

		iStart++;
		iEnd--;
	}

	cout << iResult;

	return 0;
}