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

const int N = 2e5 + 5;

int n;
int d[N];
int tab[N];
int rev_tab[N];
string S;
int rev[2][N];
int num[N];
int licz[2];

bool initial_check() {
	for (int i = 0; i < n; i++) {
		licz[S[i] - 'a']++;
	}
	if ((licz[0] & 1) && (licz[1] & 1)) {
		return false;
	}
	if (!(n & 1)) {
		return true;
	}
	if (licz[0] & 1) {
		return true;
	}
	for (int i = 0; i < n; i++) {
		S[i] = 'b' + 'a' - S[i];
	}
	return true;
}

void compute_rev() {
	licz[0] = licz[1] = 0;
	for (int i = 0; i < n; i++) {
		licz[S[i] - 'a']++;
		num[i] = licz[S[i] - 'a'];
		rev[S[i] - 'a'][licz[S[i] - 'a']] = i;
	}
}

void compute_tab() {
	int ind = 1;
	for (int i = 0; i < n; i++) {
		int c = S[i] - 'a';
		if (2 * num[i] > licz[c] + 1) {
			continue;
		}
		if (2 * num[i] == licz[c] + 1) {
			tab[i] = (n + 1) / 2;
			rev_tab[(n + 1) / 2] = i + 1;
			continue;
		}
		// cout << "siema " << i << endl;
		int s = rev[c][licz[c] + 1 - num[i]];
		tab[i] = ind;
		rev_tab[ind] = i + 1;
		tab[s] = n - ind + 1;
		rev_tab[n - ind + 1] = s + 1;
		ind++;
	}
	// if (n & 1) {
	// 	rev_tab[(n + 1) / 2] = 
	// }
}

void insert(int x, int ile) {
	if (x > n) {
		return;
	}
	d[x] += ile;
	insert(x + (x & (-x)), ile);
}

int sum(int x) {
	if (x <= 0) {
		return 0;
	}
	return d[x] + sum(x - (x & (-x)));
}

int sum_int(int a, int b) {
	return sum(b) - sum(a - 1);
}

int main() {
	// ios_base::sync_with_stdio(false);
	// cin.tie(0);

	cin >> S;
	n = S.length();
	if (!initial_check()) {
		cout << -1 << "\n";
		return 0;
	}
	compute_rev();
	compute_tab();
	// for (int i = 0; i < n; i++) {
	// 	cout << tab[i] << " ";
	// }
	// cout << endl;
	long long res = 0;
	for (int i = 1; i <= n; i++) {
		insert(i, 1);
	}
	for (int i = 1; i <= n; i++) {
		res += sum(rev_tab[i] - 1);
		insert(rev_tab[i], -1);
	}
	cout << res << "\n";
}