#include <bits/stdc++.h>
using namespace std;
int inf = (1e9) + 7;
int mod = 998244353;
int occ[26];
int main() {
  string s;
  cin >> s;
  for (int i = 0; i < s.size(); ++i) {
    occ[s[i] - 'a']++;
  }
  long long ans = 0;
  for (int i = 0; i < 26; ++i) {
    if (occ[i] % 2 == 1) ans++;
  }
  printf("%lld\n", max(ans, 1LL));
}
