#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  vector<int> v(26, 0);
  for (int i = 0; i < s.size(); i++) v[s[i] - 'a']++;
  int sum = 0, ki_size = 0;
  int ans = 0;
  for (int i = 0; i < 26; i++) {
    if (v[i] == 0) continue;
    if (!(v[i] % 2))
      sum += v[i] / 2;
    else {
      sum += (v[i] / 2);
      ki_size++;
    }
  }
  if (ki_size == 0)
    cout << "1" << endl;
  else
    cout << ki_size << endl;
}
