String tmp = new Scanner(System.in).next();
		int[] map = new int[26];
		for(int i=0;i<tmp.length();i++){
			int c = tmp.charAt(i)-'a';
			map[c]++;
		}
		int count1 = 0;
		for(int i=0;i<26;i++)
			if(map[i]!=0 && (map[i]&1) == 1)
				count1++;
		System.out.println(count1);