#include <bits/stdc++.h>
using namespace std;
int main(int argc, char *argv[argc]) {
  int hist[26] = {0};
  char c;
  while (scanf("%c", &c)) {
    if (c == '\n') break;
    hist[c - 'a']++;
  }
  int odd = 0;
  for (int i = 0; i < int(26); i++) {
    if (hist[i] % 2 == 1) odd++;
  }
  printf("%d\n", ((odd) >= (1) ? (odd) : (1)));
  return 0;
}
