#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const long long LINF = 1e18;
template <typename T>
void fin(T a) {
  cout << a << endl;
  exit(0);
}
signed main() {
  string s;
  cin >> s;
  std::vector<long long> v(26, 0);
  for (char p : s) v[p - 'a'] ^= 1;
  long long res = 0;
  for (long long i = 0; i < 26; i++) res += v[i];
  fin(max(res, 1LL));
}
