#include <bits/stdc++.h>
using namespace std;
int main() {
  int count = 0;
  vector<pair<char, int> > v;
  string str;
  cin >> str;
  v.push_back(make_pair(str[0], 1));
  for (int i = 1; i < str.length(); i++) {
    bool f = false;
    for (int j = 0; j < v.size(); j++) {
      if (v.at(j).first == str[i]) {
        v.at(j).second++;
        f = true;
        break;
      }
    }
    if (!f) v.push_back(make_pair(str[i], 1));
  }
  for (int i = 0; i < v.size(); i++) {
    count += v.at(i).second % 2;
  }
  cout << count << endl;
}
