#include <bits/stdc++.h>
using ll = long long;
using namespace std;
using bs = bitset<26>;
int main() {
  string s;
  cin >> s;
  int len = s.size();
  bs rui;
  for (int i = 0; i < len; i++) rui.flip(s[i] - 'a');
  cout << max((int)rui.count(), 1) << '\n';
  return 0;
}
