#include <bits/stdc++.h>
using namespace std;
const long long oo = 0x3f3f3f3f3f3f3f3f;
const double eps = 1e-9;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  vector<long long> cnt(26);
  for (char c : s) cnt[c - 'a']++;
  long long res = 0;
  for (long long i = (0); i < (26); i++) res += cnt[i] % 2;
  if (res == 0) res++;
  cout << res << endl;
}
