#include <bits/stdc++.h>
using namespace std;
unsigned long long over = 1000000007;
int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed;
  string s;
  cin >> s;
  int az[26];
  fill(az, az + 26, 0);
  for (size_t i = 0; i < s.size(); ++i) az[s[i] - 'a']++;
  bool isone[26];
  for (size_t i = 0; i < 26; ++i) {
    isone[i] = (az[i] == 1 ? true : false);
  }
  int azp[26];
  fill(azp, azp + 26, 0);
  azp[s[0] - 'a'] = 1;
  int ans = 0;
  int ansp;
  for (size_t i = 1; i < s.size(); ++i) {
    if (isone[s[i - 1] - 'a'] && isone[s[i] - 'a']) {
      ansp = 0;
      for (size_t j = 0; j < 26; ++j) {
        if (azp[j] % 2 == 1) ansp++;
      }
      if (ansp == 0) ansp = 1;
      ans += ansp;
      fill(azp, azp + 26, 0);
    }
    azp[s[i] - 'a']++;
  }
  ansp = 0;
  for (size_t j = 0; j < 26; ++j) {
    if (azp[j] % 2 == 1) ansp++;
  }
  if (ansp == 0) ansp = 1;
  ans += ansp;
  cout << ans << endl;
  return 0;
}
