#include <bits/stdc++.h>
using namespace std;
static const int INF = (1 << 21);
int main() {
  string S;
  int c[26];
  cin >> S;
  for (int i = 0; i < S.length(); i++) {
    c[(int)(S[i] - 97)]++;
  }
  int ans = 0;
  for (int i = 0; i < 26; i++) {
    ans += (c[i] % 2);
  }
  if (ans == 0) ans++;
  if (ans >= 2) ans--;
  cout << ans << endl;
}
