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