#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  vector<int> cnt(26);
  for (int i = 0; i < s.size(); ++i) cnt[s[i] - 'a']++;
  int ans = 0;
  for (int i = 0; i < 26; ++i)
    if (cnt[i] % 2 != 0) ans++;
  cout << max(ans, 1) << endl;
  return 0;
}
