#include <bits/stdc++.h>
using namespace std;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  long al[26];
  memset(al, 0, sizeof(al));
  string s;
  cin >> s;
  for (int i = 0; i < (long)s.size(); i++) {
    al[s[i] - 'a']++;
  }
  int ans = 0;
  for (int i = 0; i < 26; i++) {
    if (al[i] % 2 != 0) {
      ans++;
    }
  }
  if (ans == 0) ans = 1;
  cout << ans << endl;
  return 0;
}
