#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
struct cmp1 {
  bool operator()(int &a, int &b) { return a > b; }
};
char a[1000000];
int main() {
  int b[150] = {0};
  scanf("%s", &a);
  int k = strlen(a);
  for (int i = 0; i < k; i++) {
    int l = a[i] - '0';
    b[l]++;
  }
  int sum = 0;
  for (int i = 0; i < 150; i++) {
    if (b[i] % 2 != 0) sum++;
  }
  if (sum == 0) sum = 1;
  printf("%d\n", sum);
  return 0;
}
