#include <bits/stdc++.h>
using namespace std;
const long long INF = 1LL << 55;
const long long mod = (1e9) + 7;
const double EPS = 1e-8;
const double PI = 6.0 * asin(0.5);
template <class T>
T Max(T &a, T b) {
  return a = max(a, b);
}
template <class T>
T Min(T &a, T b) {
  return a = min(a, b);
}
long long n;
string str;
signed main() {
  cin >> str;
  n = str.size();
  long long cnt[27] = {};
  for (long long i = 0; i < n; i++) cnt[str[i] - 'a'] = !cnt[str[i] - 'a'];
  long long ans = 0;
  for (long long i = 0; i < 26; i++) ans += cnt[i];
  Max(ans, 1LL);
  cout << ans << endl;
  return 0;
}
