#include <bits/stdc++.h>
using namespace std;
int n, i, cnt[290], res;
char c;
int main() {
  while (cin >> c) {
    cnt[c]++;
  }
  for (i = 1; i <= 270; i++) res += (cnt[i] & 1);
  cout << max(1, res) << endl;
  return 0;
}
