#include <iostream>
#include <map>
#include <cstring>
using namespace std;

int main() {
  char str[1000];
  map<char, int> count_alphabet;

  cin >> str;
  int str_len = strlen(str);
  // count_alphabet['a'] = 100;
  // cout << count_alphabet['a'] << endl;
  for (int i = 0; i < str_len; i++) {
    count_alphabet[ str[i] ]++;
  }

  int count = 0;
  for (auto itr : count_alphabet) {
    if (itr.second % 2 == 1) {
      count++;
    }
  }
  int count_div_2 = count / 2;

  cout << count_div_2 << endl;
}