#include <bits/stdc++.h>
using namespace std;
long long ans = 30;
int main() {
  string s;
  cin >> s;
  vector<long long> v(s.size() + 1, 0);
  for (long long i = 0; i < (long long)(s.size()); i++) {
    v[i + 1] = v[i] ^ (1LL << (s[i] - 'a'));
  }
  ans = __builtin_popcountll(v[s.size()]);
  if (ans == 0) ans = 1;
  cout << ans << endl;
  return 0;
}
