import java.util.*;
import java.text.DecimalFormat;

public class Main
{
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		String S = sc.nextLine();
		int counter = 0;

		for (int i = 0; i < (S.length()/2); i++)
		{
			if (S.charAt(i) != S.charAt(S.length()-(i+1)))
				counter++;
		}

		System.out.println(counter);
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz