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#include <bits/stdc++.h>
using namespace std;

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	
	string s;
	cin >> s;
	
	int count_a = 0, count_b = 0;
	for (auto c : s) {
		if (c == 'a') {
			count_a++;
		}
		else {
			count_b++;
		}
	}
	
	if (count_a % 2 == 1 && count_b % 2 == 1) {
		cout << -1 << '\n';
		return 0;
	}
	
	if (count_a % 2 == 1) {
		for (auto &c : s) {
			if (c == 'a') {
				c = 'b';
			}
			else {
				c = 'a';
			}
		}
	}
	
	vector<int> positions_a;
	for (int i = 0; i < (int) s.size(); i++) {
		if (s[i] == 'a') {
			positions_a.emplace_back(i);
		}
	}
	
	long long result = 0;
	for (int left = 0, right = positions_a.size() - 1; left < right; left++, right--) {
		int left_position = positions_a[left], right_position = s.size() - 1 - positions_a[right];
		result += abs(left_position - right_position);
	}
	cout << result << '\n';
	
	return 0;
}