#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
const ll mod = 1e9 + 7;
const ll INF = 1e9;
int main() {
  cin.sync_with_stdio(false);
  string s;
  cin >> s;
  int c[26] = {};
  for (int i = 0; i < (int)(s.size()); i++) {
    c[s[i] - 'a']++;
  }
  int ans = 0;
  for (int i = 0; i < (int)(26); i++) {
    if (c[i] % 2 != 0) ans++;
  }
  if (ans < 2)
    cout << 1 << endl;
  else
    cout << ans << endl;
  return 0;
}
