#include <bits/stdc++.h>
using namespace std;
int las[70000000];
int main() {
  int len = 0, p = 0;
  for (char ch = getchar(); isalpha(ch); ch = getchar())
    p = p ^ (1 << (ch - 'a')), las[p] = ++len;
  int now = 0, ans = 0, val = 0;
  while (now < len) {
    int be = las[val], lasval = val;
    for (int i = 0; i < 26; i++)
      if (las[lasval ^ (1 << i)] > be) {
        val = lasval ^ (1 << i);
        be = las[val];
      }
    now = be;
    ans++;
  }
  printf("%d\n", ans);
  return 0;
}
