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