#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  int i;
  cin >> S;
  int a = S.size();
  int flag[26];
  int ans = 0;
  for (i = 0; i < 26; i++) flag[i] = 0;
  for (int j = 0; j < a; j++) {
    if (flag[S[j] - 'a'] == 0)
      flag[S[j] - 'a'] = 1;
    else
      flag[S[j] - 'a'] = 0;
  }
  for (i = 0; i < 26; i++) ans += flag[i];
  if (ans == 0) ans = 1;
  cout << ans << endl;
  return 0;
}
