#include <bits/stdc++.h>
using namespace std;
char str[200010] = {0};
int main() {
  int bj[26] = {0};
  scanf("%s", str);
  int i;
  for (i = 0; str[i] != '\0'; i++) {
    bj[str[i] - 'a']++;
  }
  int s = 0;
  for (i = 0; i < 26; i++) {
    if (bj[i] & 1) s++;
  }
  printf("%d\n", s);
  return 0;
}
