#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  string s;
  cin >> s;
  vector<int> cnt(26, 0);
  for (char c : s) {
    ++cnt[c - 'a'];
  }
  int res = 0;
  for (int x : cnt) {
    if (x % 2 == 1) {
      ++res;
    }
  }
  if (res == 0) ++res;
  cout << res << endl;
  return 0;
}
