#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, num = 0;
  char a[200005] = {0};
  int b[30] = {0};
  cin >> a;
  i = 0;
  while (a[i]) i++;
  n = i;
  for (i = 0; i < n; i++) b[a[i] - 96]++;
  for (i = 1; i <= 26; i++)
    if (b[i] % 2 != 0) num++;
  if (num == 0) num = 1;
  cout << num << endl;
  return 0;
}
