#include <bits/stdc++.h>
using namespace std;
class Solver {
 private:
  string str;
  unordered_map<int, unordered_map<char, int> > um;

 public:
  Solver() { cin >> str; }
  void exec() {
    for (int i = 0; i < str.size(); ++i) {
      um[i][str[i]]++;
      if (i != str.size() - 1) {
        um[i + 1] = um[i];
      }
    }
    int ans = 0;
    for (auto itr : um[str.size() - 1]) {
      if (itr.second % 2 == 1) {
        ans++;
      }
    }
    cout << ans << endl;
  }
};
int main() {
  Solver ce = Solver();
  ce.exec();
}
