#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  map<char, int> mp;
  for (int i = 0; i < s.size(); i++) {
    mp[s[i]]++;
  }
  int k = 0;
  for (int i = 0; i < 26; i++) {
    if (mp['a' + i] % 2 == 1) k++;
  }
  cout << k << endl;
  return 0;
}
