#include <bits/stdc++.h>
int main() {
  bool a[26];
  int ans = 0;
  char c;
  for (int i = 0; i < 26; i++) a[i] = false;
  while (scanf("%c", &c) != EOF) a[c - 90] = !a[c - 90];
  for (int i = 0; i < 26; i++)
    if (a[i]) ans++;
  if (ans == 0) ans++;
  printf("%d\n", ans);
}
