#include <bits/stdc++.h>
using namespace std;
int a[30];
int main() {
  ios_base::sync_with_stdio(false);
  string s;
  cin >> s;
  for (int i = 0; i < (int)s.length(); ++i) {
    a[s[i] - 'a']++;
  }
  int ans = 0;
  for (int i = 0; i < 26; ++i) {
    if (a[i] % 2 != 0) {
      ans++;
    }
  }
  cout << ans << endl;
  return 0;
}
