#include <bits/stdc++.h>
using namespace std;
const long long mod = 1000000007;
const long long INF = (long long)1000000007 * 1000000007;
const long double eps = 1e-8;
const long double pi = acos(-1.0);
int n;
string s;
void solve() {
  cin >> s;
  n = s.length();
  int ord_a = 'a';
  int ans = 0;
  for (int i = 0; i < 26; i++) {
    char t = ord_a + i;
    int k = 0;
    for (int i = 0; i < n; i++) {
      if (s[i] == t) k += 1;
    }
    if (k % 2 == 1) ans += 1;
  }
  cout << max(1, ans) << endl;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(50);
  solve();
}
