#include <bits/stdc++.h>
using namespace std;
using namespace std::chrono;
const long long big = 1000000007ll;
const long long big2 = 1000000009ll;
long long n, m, k, q, w, h, T;
int main() {
  long long a, b, c, d;
  string s;
  cin >> s;
  long long F[26] = {0};
  n = s.length();
  for (long long c1 = 0; c1 < n; c1++) {
    F[s[c1] - 'a']++;
  }
  long long ans = 0;
  for (long long c1 = 0; c1 < 26; c1++) {
    ans += F[c1] % 2;
  }
  if (ans == 0) ans++;
  cout << ans << "\n";
  return 0;
}
