#include <bits/stdc++.h>
using namespace std;
long long int N, a[26], i;
char s[364364];
int main() {
  scanf("%s", s);
  N = strlen(s);
  for (int i = 0; i < N; ++i) a[s[i] - 97] ^= 1;
  cout << accumulate(a, a + 26, 0) << endl;
  return 0;
}
