#include <bits/stdc++.h>
using namespace std;
int main(void) {
  string s;
  cin >> s;
  vector<int> cnt(26, 0);
  for (char ch : s) {
    ++cnt[ch - 'a'];
  }
  int odd = 0, even = 0;
  for (int i = 0; i < 26; ++i) {
    if (cnt[i] % 2 == 1)
      ++odd;
    else if (cnt[i] > 0)
      ++even;
  }
  cout << max(odd, 1) << endl;
  return 0;
}
