#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int cnt[26] = {0};
  for (int i = 0; i < s.size(); ++i) {
    int temp = (int)(s[i] - 'a');
    ++cnt[temp];
  }
  int ans = 0;
  int odd = false;
  for (int i = 0; i < 26; ++i) {
    if (cnt[i] > 0) {
      if (cnt[i] % 2 == 0) {
        cnt[i] == 0;
      } else if (odd == false) {
        cnt[i] == 1;
      }
    }
  }
  ++ans;
  for (int i = 0; i < 26; i++) {
    if (cnt[i] % 2 == 1) +ans;
  }
  cout << ans << endl;
}
