#include <bits/stdc++.h>
using namespace std;
template <typename X>
bool exist(vector<X> vec, X item) {
  return find(vec.begin(), vec.end(), item) != vec.end();
}
long long alb[26];
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string str;
  cin >> str;
  for (long long i = (long long)0; i < (long long)str.size(); i++) {
    alb[str[i] - 'a']++;
  }
  long long ans = 0;
  for (long long i = (long long)0; i < (long long)26; i++) {
    if ((alb[i] % 2 == 1)) {
      ans++;
    }
  }
  if (ans == 0)
    cout << 1 << endl;
  else
    cout << ans << endl;
  return 0;
}
