#include <bits/stdc++.h>
using namespace std;
string s;
int cnt[26][200000], cnt2[26];
int ans;
bool flg[26], tmp;
int fst, snd;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> s;
  for (int i = (0); i < (s.size()); ++i) {
    if (i != 0) {
      for (int j = (0); j < (26); ++j) cnt[j][i] = cnt[j][i - 1];
    }
    cnt[s[i] - 'a'][i]++;
    cnt2[s[i] - 'a']++;
  }
  bool ok = false;
  for (int i = (0); i < (26); ++i) {
    if (cnt2[i] % 2 == 1) ok = true;
  }
  if (!ok) {
    cout << '1' << endl;
    return 0;
  }
  for (int i = (0); i < (26); ++i) flg[i] = false;
  for (int i = (0); i < (s.size()); ++i) {
    if (cnt2[s[i] - 'a'] == 1) {
      ans++;
      if (!tmp)
        tmp = true, fst = i, snd = i;
      else {
        fst = snd, snd = i;
        for (int j = (0); j < (26); ++j) {
          if (s[i] - 'a' == j) continue;
          if ((cnt2[j] - cnt[j][fst]) % 2 == 1 && cnt2[j] != 1) {
            flg[j] = true;
            ans++;
          }
        }
      }
    }
  }
  for (int i = (0); i < (26); ++i) {
    if (cnt2[i] % 2 == 1 && cnt2[i] != 1 && !flg[i]) ans++;
  }
  cout << ans << endl;
  return 0;
}
