#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  int a[26] = {}, b = 0;
  cin >> s;
  for (int i = 0; i < s.size(); i++) a[s[i] - 'a']++;
  for (int i = 0; i < 26; i++)
    if (a[i] % 2 == 1) b++;
  if (b == 0)
    cout << 1;
  else
    cout << b;
  return 0;
}
