#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  long long K[26];
  for (long long i = 0; i < 26; i++) K[i] = 0;
  for (long long i = 0; i < s.size(); i++) {
    K[s[i] - 'a']++;
  }
  long long ans = 0;
  for (long long i = 0; i < 26; i++) {
    ans += (K[i] % 2 != 0);
  }
  cout << max(ans, (long long)1) << endl;
  return 0;
}
