import java.io.*;

class Main {
	public static void main(String[] args) {
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		try{
			String s = reader.readLine();
			char[] c = s.toCharArray();
			int n = c.length;
			int ans = 0;
			if(n % 2 == 0){
				for(int i = 0; i < (n / 2); i++){
					if(c[i] != c[(n - 1) - i]){
						ans ++;
					}
				}
			}else{
			for(int i = 0; i < (n - 1) / 2; i++){
				if(c[i] != c[(n - 1) - i]){
					ans ++;
				}
			}
		}
		System.out.println(ans);
		} catch(IOException e) {
			System.out.println(e);
		} 
	}
}
/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