#include <bits/stdc++.h>
using namespace std;
set<char> sc;
int main() {
  ios::sync_with_stdio(false);
  cin.tie();
  cout.tie();
  string s;
  cin >> s;
  long long a[1000];
  memset(a, 0, sizeof(a));
  for (int i = 0; i < s.size(); i++) {
    a[s[i]]++;
    sc.insert(s[i]);
  }
  set<char>::iterator it;
  long long numjs = 0, numos = 0;
  for (it = sc.begin(); it != sc.end(); it++) {
    if (a[*it] % 2 == 0)
      numos++;
    else
      numjs++;
  }
  if (numjs == 0 || numjs == 1 && numos != 0)
    cout << "1" << endl;
  else if (numos == 0)
    cout << numjs << endl;
  else {
    if (numjs > numos)
      cout << numos + (numjs - numos) << endl;
    else if (numjs == numos)
      cout << numos << endl;
    else
      cout << numjs << endl;
  }
  return 0;
}
