#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  map<char, int> hist;
  for (long long i = 0; i < (long long)(s.size()); i++) {
    hist[s[i]]++;
  }
  int sum = 0;
  for (auto e : hist) {
    if (e.second % 2) sum++;
  }
  if (!sum)
    cout << 1 << endl;
  else
    cout << sum << endl;
}
