#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  string abc = "abcdefghijklmnopqrstuvwxyz";
  cin >> s;
  long long int d[26] = {0};
  for (long long int i = 0; i < s.size(); i++) {
    for (int j = 0; j < 26; j++) {
      if (s[i] == abc[j]) {
        d[j]++;
      }
    }
  }
  long long int ans = 0;
  for (int i = 0; i < 26; i++) {
    if (d[i] % 2 == 1) {
      ans++;
    }
  }
  if (ans == 0) {
    ans = 1;
  }
  cout << ans << endl;
  return 0;
}
