#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-9;
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  string s;
  cin >> s;
  long long n = s.size();
  vector<long long> tmp(26, 0);
  for (long long i = (0); i < (n); i++) tmp[s[i] - 'a']++;
  long long ans = 0;
  for (long long i = (0); i < (26); i++)
    if (tmp[i] % 2) ans++;
  cout << max(ans, 1LL) << endl;
  return 0;
}
