#include <bits/stdc++.h>
int main(void) {
  char s[200000];
  int i, c[26], l, count = 0;
  for (i = 0; i < 26; i++) c[i] = 0;
  scanf("%s", s);
  l = strlen(s);
  for (i = 0; i < l; i++) c[s[i] - 'a']++;
  for (i = 0; i < 26; i++)
    if (c[i] % 2 == 1) count++;
  printf("%d\n", count);
  return 0;
}
