#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1000005;
char str[MAXN];
int dict[60];
int main() {
  while (~scanf("%s", str)) {
    memset(dict, 0, sizeof(dict));
    for (int i = 0; i < strlen(str); ++i) dict[str[i] - 'a']++;
    int ans = 0;
    for (int i = 0; i < 26; ++i)
      if (dict[i] & 1) ans++;
    if (ans)
      printf("%d\n", ans);
    else
      printf("1\n");
  }
  return 0;
}
