#include <bits/stdc++.h>
using namespace std;
int main() {
  char a = 'a';
  long long atoz[26] = {0};
  long long ans = 0;
  char c;
  while (cin >> c) {
    atoz[c - a]++;
  }
  for (int i = 0; i < 26; i++) {
    if (atoz[i] % 2 == 1) {
      ans++;
    }
  }
  if (ans == 0) {
    cout << 1 << endl;
  } else {
    cout << ans << endl;
  }
  return 0;
}
