#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  map<char, int> M;
  for (int i = 0; i < S.length(); ++i) {
    ++M[S[i]];
  }
  int ans = 0;
  for (map<char, int>::iterator itr = M.begin(); itr != M.end(); ++itr) {
    ans += itr->second % 2;
  }
  cout << max(ans, 1) << endl;
  return 0;
}
