#include <bits/stdc++.h>
using namespace std;
struct edge {
  int from, to;
  long long cost;
};
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  string s;
  cin >> s;
  int n = (int)s.length();
  map<char, int> mp;
  for (int i = 0; i < n; i++) {
    mp[s[i]]++;
  }
  long long ans = 0LL;
  for (auto iter = mp.begin(); iter != mp.end(); ++iter) {
    if (iter->second % 2 == 1) {
      ans++;
    }
  }
  cout << max(1LL, ans);
}
