#include <bits/stdc++.h>
using namespace std;
const unsigned long long mod = 1e9 + 7;
template <class S, class T>
ostream& operator<<(ostream& os, const pair<S, T> v) {
  os << "(" << v.first << ", " << v.second << ")";
  return os;
}
template <class T>
ostream& operator<<(ostream& os, const vector<T> v) {
  for (int i = 0; i < (int)v.size(); i++) {
    if (i > 0) {
      os << " ";
    }
    os << v[i];
  }
  return os;
}
template <class T>
ostream& operator<<(ostream& os, const vector<vector<T>> v) {
  for (int i = 0; i < (int)v.size(); i++) {
    if (i > 0) {
      os << endl;
    }
    os << v[i];
  }
  return os;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  vector<long long> cnt(26, 0);
  for (int i = 0; i < (int)s.size(); ++i) {
    cnt[s[i] - 'a']++;
  }
  long long odd = 0;
  for (int i = 0; i < (int)26; ++i)
    if (cnt[i] % 2) odd++;
  cout << (odd == 0 ? 1 : odd) << endl;
  return 0;
}
