#include <bits/stdc++.h>
using namespace std;
const long long INF = 1107110711071107;
signed main() {
  string s;
  cin >> s;
  long long cnt[26] = {0};
  long long n = s.size();
  for (long long i = 0; i < (n); i++) cnt[s[i] - 'a']++;
  long long ans = 0;
  for (long long i = 0; i < (26); i++) ans += cnt[i] % 2;
  cout << ans << endl;
}
