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