#include <bits/stdc++.h>
using namespace std;
int main() {
  char w;
  int a[26] = {};
  while (cin >> w) {
    a[w - 'a'] = a[w - 'a'] + 1;
  }
  int x = 0;
  for (int i = 0; i < 26; i++) {
    if (a[i] % 2 == 1) {
      x = x + 1;
    }
  }
  if (x > 0) {
    cout << x << endl;
  } else {
    cout << 1 << endl;
  }
}
