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