#include <bits/stdc++.h>
using namespace std;
int main(void) {
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  vector<bool> parity(26, false);
  for (int i = 0; i < s.size(); i++) parity[s[i] - 'a'] = !parity[s[i] - 'a'];
  int ans = 0;
  for (int i = 0; i < 26; i++) ans += parity[i];
  cout << max(1, ans) << endl;
  system("pause");
  return 0;
}
