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

inline bool is_palindrome_impossible(const string& str) {
	unsigned long long a = 0, b = 0;
	for(const char& i : str) {
		if(i == 'a')
			++a;
		else
			++b;
	}
	return ((a&1) && (b&1));
}

bool is_palindrome(string::const_iterator a, string::const_iterator b) {
	while(a < --b)
		if(*(a++) != *b)
			return false;
	return true;
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);

	string str;
	cin >> str;

	if(is_palindrome_impossible(str)) {
		cout << "-1";
		return 0;
	}

	unsigned long long answer = 0;
	auto iter_a = str.begin(), iter_b = --str.end();

	while(iter_a < iter_b) {
		auto tmp_a = iter_a, tmp_b = iter_b;
		while(*tmp_a != *tmp_b)
			--tmp_b;

		if(tmp_a == tmp_b) {
			swap(*tmp_b, *(tmp_b+1));
			++answer;
		} else {
			swap(*tmp_b, *iter_b);
			answer += iter_b - tmp_b;
			++iter_a;
			--iter_b;
		}
	}

	cout << answer;
}