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#include <cstring>
#include <cstdio>
#include <cstdlib>


using namespace std;

int findNextR(char* pal, char letter, int start, int len)
{
	while (start < len && pal[start] != letter)
	{
		start++;
	}

	return start;
}

int findNextL(char* pal, char letter, int start)
{
	while (start >= 0 && pal[start] != letter)
	{
		start--;
	}

	return start;
}

int main()
{
	char* pal = new char[200001];

	scanf("%s", pal);
	int l = strlen(pal);
	int a = 0, b = 0;
	for (int i = 0; i < l; i++)
	{
		if (pal[i] == 'a')
		{
			a++;
		}
		else
		{
			b++;
		}
	}

	if (((l % 2 == 0) && (a % 2 || b % 2)))
	{
		printf("%d", -1);
		return 0;
	}

	int left = 0, right = l - 1, nextAl = findNextR(pal, 'a', 0, l), nextAr = findNextL(pal, 'a', l - 1),
		nextBl = findNextR(pal, 'b', 0, l), nextBr = findNextL(pal, 'b', l - 1), res = 0;

	while (left < right)
	{
		if (pal[left] != pal[right])
		{
			nextAl = (nextAl < left) ? findNextR(pal, 'a', left, l) : nextAl;
			nextBl = (nextBl < left) ? findNextR(pal, 'b', left, l) : nextBl;
			nextAr = (nextAr > right) ? findNextL(pal, 'a', right) : nextAr;
			nextBr = (nextBr > right) ? findNextL(pal, 'b', right) : nextBr;
			
			if(pal[left] == 'a')
			{
				if (nextBl - left < right - nextAr)
				{
					pal[left] = 'b';
					pal[nextBl] = 'a';
					res += nextBl - left;
					nextAr = nextBl > nextAr ? nextBl : nextAr;
					nextBl = findNextR(pal, 'b', nextBl, l);
					

				}
				else
				{
					pal[right] = 'a';
					pal[nextAr] = 'b';
					res += right - nextAr;				
					nextBl = nextAr < nextBl ? nextAr : nextBl;
					nextAr = findNextL(pal, 'a', nextAr);
				}
			}
			else
			{
				if (nextAl - left < right - nextBr)
				{
					pal[left] = 'a';
					pal[nextAl] = 'b';
					res += nextAl - left;
					nextBr = nextAl > nextBr ? nextAl : nextBr;
					nextAl = findNextR(pal, 'a', nextAl, l);
				}
				else
				{
					pal[right] = 'b';
					pal[nextBr] = 'a';
					res += right - nextBr;
					nextAl = nextBr < nextAl ? nextBr : nextAl;
					nextBr = findNextL(pal, 'b', nextBr);
				}
			}

		}

		left++; right--;
	}

	printf("%d", res);

  	return 0;
}