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