#include <bits/stdc++.h>
using namespace std;
int main() {
  string a;
  cin >> a;
  int n = a.length();
  int res = 0;
  int c[26] = {0};
  for (int i = 0; i < n; ++i) {
    c[a[i] - 97]++;
  }
  for (int i = 0; i < 26; ++i) {
    if (c[i] & 1) ++res;
  }
  if (res == 0) {
    cout << 1;
  } else {
    cout << res;
  }
  return 0;
}
