#include <bits/stdc++.h>
using namespace std;
bool vdp[200050];
char buf[200050];
int n, bkt[255];
int main() {
  scanf("%s", buf);
  n = strlen(buf);
  for (int i = 0; i < n; i++) {
    bkt[buf[i]]++;
  }
  int ans = 0;
  for (int i = 0; i < 255; i++) {
    if (bkt[i] % 2 == 1) ans++;
  }
  printf("%i\n", max(ans, 1));
}
