#include <bits/stdc++.h>
int main() {
  char s[200000] = {};
  scanf("%s", s);
  int ls = 0;
  for (int i = 0; 0 == 0; i++) {
    if (s[i] == 0) {
      ls = i;
      break;
    }
  }
  std::sort(s, s + ls);
  int k = 0;
  int l = 0;
  int ans = 0;
  for (int i = 0; i < ls; i++) {
    if (k != s[i]) {
      if (l % 2 == 1) {
        ans++;
      }
      l = 0;
      k = s[i];
      l++;
    } else {
      l++;
    }
    if (i == ls - 1) {
      if (l % 2 == 1) ans++;
    }
  }
  if (ans == 0) {
    ans = 1;
  }
  printf("%d", ans);
  return 0;
}
