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