#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e9 + 7;
const long long mod = 998244353;
signed main() {
  string s;
  cin >> s;
  vector<long long> cnt(26);
  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++) {
    if (cnt[i] % 2) ans++;
  }
  cout << ans << '\n';
}
