#include <bits/stdc++.h>
using namespace std;
long long solve(string s) {
  if (s == "") return 0;
  long long K[26];
  for (long long i = 0; i < 26; i++) K[i] = 0;
  for (long long i = 0; i < s.size(); i++) {
    K[s[i] - 'a']++;
  }
  char element = 'a';
  bool res = false;
  for (long long i = 0; i < 26; i++) {
    if (K[i] == 1) {
      element = 'a' + i;
      res = true;
      break;
    }
  }
  if (res) {
    string s1 = "", s2 = "";
    long long u = 0;
    while (s[u] != element) {
      s1 += s[u];
      u++;
    }
    u++;
    while (u < s.size()) {
      s2 += s[u];
      u++;
    }
    return 1 + solve(s1) + solve(s2);
  }
  long long ans = 0;
  for (long long i = 0; i < 26; i++) {
    ans += (K[i] % 2 != 0);
  }
  return max(ans, (long long)1);
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  cout << solve(s);
  return 0;
}
