#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[200010], a[200] = {0};
  scanf("%s", s);
  for (int i = 0; s[i] != '\0'; i++) {
    a[s[i]]++;
  }
  int sum = 0;
  for (int i = 'a'; i <= 'z'; i++) {
    if (a[i] % 2 != 0) sum++;
  }
  if (sum == 0) sum = 1;
  printf("%d\n", sum);
  return 0;
}
