#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int i, j;
  string s;
  cin >> s;
  int n = s.size();
  vector<int> alph(26, 0);
  for (i = 0; i < n; i++) alph[s[i] - 'a']++;
  int ans = 0;
  for (i = 0; i < 26; i++)
    if (alph[i] % 2) ans++;
  if (ans == 0) ans++;
  cout << ans << endl;
}
