#include <bits/stdc++.h>
using namespace std;
int main() {
  int bucket[26] = {0};
  int n = 0;
  string s;
  cin >> s;
  for (int i = 0; i < (int)s.size(); i++) {
    if (bucket['z' - s[i]] & 1)
      bucket['z' - s[i]] = 0;
    else
      bucket['z' - s[i]] = 1;
  }
  for (int i = 0; i < 26; i++) {
    if (bucket[i] & 1) n++;
  }
  if (n == 0) n = 1;
  cout << n << endl;
  return 0;
}
