#include <bits/stdc++.h>
using namespace std;
const int MX = 200000;
char s[MX + 1];
int cnt[26];
int main() {
  ignore = scanf("%s", s);
  for (int i = 0; s[i] != 0; i++) cnt[s[i] - 'a'] ^= 1;
  int ans = 0;
  for (int i = 0; i < 26; i++) ans += cnt[i];
  printf("%d\n", ans);
  return 0;
}
