import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Scanner;

//制造回文

public class MakePlalindrome {

	public static void main(String[] args) {
		int result = 0;
		Map<Character, Integer> dic = new HashMap<Character, Integer>();
		Scanner input = new Scanner(System.in);
		String str = input.nextLine();
		char[] chars = str.toCharArray();
		for (char c : chars) {
			if(!dic.containsKey(c)){
				dic.put(c, 1);
			}else{
				dic.put(c, dic.get(c)+1);
			}
		}
//		System.out.println(dic.toString());
		
		Iterator iter = dic.entrySet().iterator();	
		while (iter.hasNext()) {
			Map.Entry entry = (Map.Entry) iter.next();
			int val = (int) entry.getValue();
			if(val%2 == 1){
				result++;
			}
		}
		System.out.println(result);
	}
}