#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  char s[200100];
  scanf("%s", s);
  n = strlen(s);
  int a[30] = {0};
  for (int i = (long long int)0; i < n; i++) {
    int e = s[i] - 'a';
    a[e]++;
  }
  int r = 0;
  for (int i = (long long int)0; i < 26; i++) {
    if (a[i] % 2 == 1) {
      r++;
    }
  }
  printf("%d\n", max(1, r));
  return 0;
}
