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#include <stdio.h>
#include <time.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <map> 
#include <string>
#include <vector>  
#include <iostream> 
#include <sstream> 
#include <queue>
#include <algorithm>
#include <assert.h>


using namespace std;

#define ll long long
#define PB 		push_back
#define FOR(a,start,end) 	for(int a=int(start); a<int(end); a++)
#define INF 		INT_MAX
#define SORT(a) 	sort(a.begin(),a.end()) 
#define CL(a,x) 		memset(a,x,sizeof(a))
#define REP(a,x)	for(int a=0;a<x;a++)
#define REP1(a,x)	for(int a=1;a<=x;a++)
#define MP 		make_pair



typedef vector<int>     vi;
typedef pair<ll, ll>     pii;
typedef vector<string> vs;
typedef vector<pii> vii;
typedef vector<vector<ll> > vvi;
typedef vector<vector<string> > vvs;
typedef vector<pair<string, string> > vss;
typedef pair<string, string> pss;
typedef pair<ll, pii> ppii;
typedef vector<ppii> vppii;
typedef vector<vector<pii> > vvii;
typedef vector<vvi> vvvi;


int b1, n, k, a, b, k1, k2,z;
vi vk1, vk2;
string wa, wb, wc;


int main() {
	//freopen("c:\\wojtek\\uva\\pa\\debug\\t1.in.txt", "rt", stdin);

	//while (1) {

	cin >> wa;
	vk1.clear();
	vk2.clear();
	k = wa.size();
	k1 = 0;
	k2 = 0;
	z = 0;
	for (int i = 0; i < k; i++) {
		if (wa[i] == 'a') {
			vk1.push_back(i);
		}
		else {
			vk2.push_back(i);
		}
	}
	k1 = vk1.size();
	k2 = vk2.size();
	if (k % 2 == 0) {
		if (k1 % 2) {
			z = -1;
		}
		else {
			for (int i = 0; i < k1/2; i++) {
				z += abs(vk1[i] - (k-1 - vk1[k1 - i - 1]));
			}
		}
	}
	else {
		if (k1 % 2) {
			for (int i = 0; i < k1 / 2; i++) {
				z += abs(vk1[i] - (k-1 - vk1[k1 - i - 1]));
			}
			z += abs(vk1[k1 / 2] - k / 2);
		}
		else {
			for (int i = 0; i < k2 / 2; i++) {
				z += abs(vk2[i] - (k-1 - vk2[k2 - i - 1]));
			}
			z += abs(vk2[k2 / 2] - k / 2);
		}
	}


	cout << z << endl;
	// }

	return 0;

}