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// pal
#include <bits/stdc++.h>
#include <cstdio>
#include <iostream>

//#define TEST

using namespace std;

#define FAST                                                                   \
  ios_base::sync_with_stdio(0);                                                \
  cin.tie(0);

#define pb push_back
#define mp make_pair

#define fi(a, b) for (int i = a; i <= b; i++)
#define fj(a, b) for (int j = a; j <= b; j++)
#define fo(a, b) for (int o = a; o <= b; o++)

#define fdi(a, b) for (int i = a; i >= b; i--)
#define fdj(a, b) for (int j = a; j >= b; j--)
#define fdo(a, b) for (int o = a; o >= b; o--)

#define sz(x) (int)x.size()

#define init_v(tab, n) fo(1, n) tab.pb(0)

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<int, int> pii;
typedef vector<pii> vpii;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<ll> vll;

template <typename T> ostream &operator<<(ostream &os, vector<T> v) {
  fi(0, sz(v) - 1) os << v[i] << " ";
  return os;
}
template <typename A, typename B>
ostream &operator<<(ostream &os, pair<A, B> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <typename T> ostream &operator<<(ostream &os, set<T> t) {
  for (auto z : t) {
    os << z << " ";
  }
  return os;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, map<T1, T2> t) {
  cerr << endl;
  for (auto z : t) {
    os << "\t" << z.first << " -> " << z.second << endl;
  }
  return os;
}

#ifdef TEST
#define dbg(x)                                                                 \
  { cerr << __LINE__ << "\t" << #x << ": " << x << endl; }
#define dbg0(x, n)                                                             \
  {                                                                            \
    cerr << __LINE__ << "\t" << #x << ": ";                                    \
    for (int ABC = 0; ABC < n; ABC++)                                          \
      cerr << x[ABC] << ' ';                                                   \
    cerr << endl;                                                              \
  }
#else
#define dbg(x) ;
#define dbg0(x, n) ;
#endif

void przekierowanie() {
#ifdef TEST
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif
}

int main() {
  FAST przekierowanie();
  vector<char> A;
  char c;
  long la = 0;
  long lb = 0;
  do {
    c = getchar();
    if (c == 'a') {
      la++;
      A.pb(c);
    }
    if (c == 'b') {
      lb++;
      A.pb(c);
    }
  } while (c == 'a' || c == 'b');

  long wynik = 0;
  auto suma = la + lb;
  if (suma < 2) {
    cout << -1 << endl;
    return 0;
  }
  if (suma % 2 == 0) { // parzyscie
    if ((la % 2)) {
      cout << -1 << endl;
      return 0;
    }
    for (long i = 0; i < A.size() / 2; i++)
      if (A[i] != A[A.size() - i - 1])
        wynik++;
    wynik /= 2;
  } else {
    auto srodek = (A.size() - 1) / 2;
    if ((la % 2 == 1 && A[srodek] == 'a') ||
        (lb % 2 == 1 && A[srodek] == 'b')) {
      for (long i = 0; i < srodek; i++)
        if (A[i] != A[A.size() - i - 1])
          wynik++;
      wynik = wynik / 2;
    } else {
      for (long i = 0; i < srodek; i++)
        if (A[i] != A[A.size() - i - 1])
          wynik++;
      wynik = ((wynik - 1) / 2) + 1;
    }
  }
  cout << wynik << endl;
  return 0;
}