#include <bits/stdc++.h>
using namespace std;
int a[200005], nw[26], pr[200005], cc[26];
int main() {
  string s;
  cin >> s;
  for (int i = 0; i < s.size(); i++) {
    pr[i] = nw[s[i] - 'a'];
    nw[s[i] - 'a'] = i;
    cc[s[i] - 'a']++;
  }
  int ans = 0;
  for (int i = 0; i < 26; i++)
    if (cc[i] & 1) ans++;
  printf("%d\n", max(1, ans));
  return 0;
}
