#include <bits/stdc++.h>
#define rep(i, a, b) for (int i = a; i <= b; i++)
#define per(i, a, b) for (int i = b; i >= a; i--)
#define cat(x) cout << #x << ": " << x << endl

using namespace std;
using ll = long long;

template<class T> struct Fenwick {
	int n;
	vector<T> f;
	Fenwick(int n) : n(n), f(n + 1) {}
	void add(int x, T v) {
		for (; x <= n; x += x & -x)
			f[x] += v;
	}
	T query(int x) {
		T res = 0;
		for (; x > 0; x -= x & -x)
			res += f[x];
		return res;
	}
	T sum(int a, int b) {
		assert(1 <= a && b <= n);
		return query(b) - query(a - 1);
	}
};

const int N = 200200;

int n, match[N];
char s[N];

int main() {
	cin.tie(0)->sync_with_stdio(0);

	cin >> s + 1;
	n = strlen(s + 1);

	vector<int> a[2];
	rep(i, 1, n) a[s[i] - 'a'].push_back(i);

	if (a[0].size() % 2 && a[1].size() % 2) {
		cout << "-1\n";
		return 0;
	}

	for (auto b : {a[1], a[0]}) {
		for (int i = 0; i < int(b.size()); i++) {
			match[b[i]] = b[int(b.size()) - 1 - i];
		}

	}

	Fenwick<int> alive(n);
	rep(i, 1, n) alive.add(i, 1);

	long long res = 0;
	rep(i, 1, n) {
		if (match[i] == i) {
			res += alive.sum(match[i] + 1, n) / 2;
		}
		if (match[i] > i) {
			res += alive.sum(match[i] + 1, n);
		}
		alive.add(match[i], -1);
	}
	cout << res << "\n";

	return 0;
}