#include <bits/stdc++.h>
using namespace std;
char a[200010];
char yi[200010];
char er[200010];
int main() {
  cin >> a;
  int len = strlen(a);
  set<int> s;
  int one = 0;
  int two = 0;
  for (int i = 0; i < len; i++) {
    if (s.count(a[i]) == 0)
      s.insert(a[i]);
    else {
      s.erase(a[i]);
      er[two++] = a[i];
    }
  }
  for (set<int>::iterator it = s.begin(); it != s.end(); it++) {
    yi[one++] = *it;
  }
  if (one == 0) one = 1;
  cout << one << endl;
  return 0;
}
