#include <bits/stdc++.h>
int main() {
  char s[200010] = {};
  long long int abc[30] = {};
  long long int len = 0, ans = 0;
  scanf("%s", s);
  len = strlen(s);
  for (long long int i = 0; i < len; i++) {
    abc[s[i] - 'a']++;
  }
  for (int i = 0; i < 26; i++) {
    if (abc[i] % 2 == 1) {
      ans++;
    }
  }
  ans = (ans == 0) ? 1 : ans;
  printf("%d", ans);
  return 0;
}
