#include <bits/stdc++.h>
template <class T>
bool maxi(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool mini(T &a, const T &b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<ll, ll>;
void Main() {
  string S;
  cin >> S;
  ll N = S.size();
  ll cha[26] = {};
  for (ll i = 0; i < N; i++) {
    ll c = S[i] - 'a';
    cha[c] = 1 - cha[c];
  }
  ll ans = 0;
  for (ll i = 0; i < 26; i++) ans += cha[i];
  cout << max(1LL, ans) << "\n";
}
int main() {
  cin.tie(nullptr);
  ios_base::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  Main();
}
