#include <bits/stdc++.h>
using namespace std;
string s;
int num[50], tot;
int f[200010];
int main() {
  ios::sync_with_stdio(false);
  cin >> s;
  for (int i = 0; i < s.length(); ++i) {
    if ((num[s[i] - 'a'] & 1) == 1)
      tot--;
    else
      tot++;
    f[i] = tot;
  }
  cout << tot << endl;
  return 0;
}
