#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int a[200], ans = 0;
  char s[200001];
  cin >> s;
  int l = strlen(s);
  for (int i = 0; i < l; i++) {
    a[int(s[i])]++;
  }
  for (int i = 'a'; i <= 'z'; i++) {
    if (a[i] % 2 == 1) ans++;
  }
  if (ans == 0) ans = 1;
  cout << ans << endl;
}
