#include <bits/stdc++.h>
using namespace std;
int main() {
  string a;
  while (cin >> a) {
    int le[27] = {0};
    int len = a.length();
    for (int i = 0; i < len; i++) {
      le[a[i] - 'a']++;
    }
    int ans = 0;
    for (int i = 0; i < 27; i++) {
      if (le[i] % 2 == 1) ans++;
    }
    cout << ans << endl;
  }
}
