#include <bits/stdc++.h>
using namespace std;
int cnt[100];
string s;
int main() {
  cin >> s;
  int n = s.size();
  for (long long i = (long long)(0); i < (long long)(n); i++) {
    cnt[s[i] - 'a']++;
  }
  int res = 0;
  for (long long i = (long long)(0); i < (long long)(26); i++) {
    if (cnt[i] % 2 == 1) res++;
  }
  if (res == 0)
    cout << 1 << endl;
  else
    cout << res << endl;
  return 0;
}
