#include <bits/stdc++.h>
using namespace std;
int fr[26];
int main() {
  string s;
  cin >> s;
  int n = s.size();
  int ans = 0;
  for (int i = 0; i < n; i++) fr[s[i] - 'a']++;
  for (int i = 0; i < 26; i++) ans += fr[i] % 2;
  if (ans == 0) ans = 1;
  cout << ans;
  return 0;
}
