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#include<cstdio>
#include<iostream>
#include<iomanip>
#include<algorithm>
#include<string>
#include<vector>
#include<cmath>
#include<set>
#include<stack>

using namespace std;

typedef vector<int> VI;
typedef long long LL;
typedef vector<LL> VLL;
typedef pair<int, int> PII;

#define FOR(x, b, e) for(int x=b; x<=(e); ++x)
#define FORD(x, b, e) for(int x=b; x>=(e); --x)
#define REP(x, n) for(int x=0; x<(n); ++x)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define PB push_back
#define ST first
#define ND second

int main() {
	ios_base::sync_with_stdio(0);
    cin.tie(NULL); cout.tie(NULL);
    string word;
    cin >> word;
    int counta = 0, countb = 0;
    for (char c : word) if (c == 'a') counta++; else countb++;
    if (counta % 2 > 0 && countb % 2 > 0) cout << "-1\n";
    else {
        int n = word.size();
        int i = 0, j = n - 1;
        int ia = i + 1, ib = i + 1;
        int ja = j - 1, jb = i - 1;
        LL result = 0;
        while (i < j) {
            if (word[i] != word[j]) {
                while(ia < n && (ia <= i || word[ia] != 'a')) ia++;
                while(ib < n && (ib <= i || word[ib] != 'b')) ib++;
                while(ja >= 0 && (ja >= j || word[ja] != 'a')) ja--;
                while(jb >= 0 && (jb >= j || word[jb] != 'b')) jb--;
                if (word[i] == 'a') {
                    if (word[ib] == 'b' && word[ja] == 'a') {
                        if (ib - i < j - ja) {
                            swap(word[i], word[ib]);
                            result += ib - i;
                            ja = max(ja, ib);
                        } else {
                            swap(word[j], word[ja]);
                            result += j - ja;
                            ib = min(ib, ja);
                        }
                    } else if (word[ib] == 'b') {
                        swap(word[i], word[ib]);
                        result += ib - i;
                        ja = max(ja, ib);
                    } else {
                        swap(word[j], word[ja]);
                        result += j - ja;
                        ib = min(ib, ja);
                    }
                } else {
                    if (word[ia] == 'a' && word[jb] == 'b') {
                        if (ia - i < j - jb) {
                            swap(word[i], word[ia]);
                            result += ia - i;
                            jb = max(jb, ia);
                        } else {
                            swap(word[j], word[jb]);
                            result += j - jb;
                            ia = min(ia, jb);
                        }
                    } else if (word[ia] == 'a') {
                        swap(word[i], word[ia]);
                        result += ia - i;
                        jb = max(jb, ia);
                    } else {
                        swap(word[j], word[jb]);
                        result += j - jb;
                        ia = min(ia, jb);
                    }
                }
            }
            i++; j--;
        }
        cout << result << '\n';
    }
    return 0;
}