#include<stdio.h>
char s[1000006];
int main()
{
	scanf("%s", s);
	int n, i, ans = 0;
	for (n = 0; s[n] != '\0'; n++);
	for (i = 0; i < n - i - 1; i++)
		if (s[i] != s[n - i - 1])
			ans++;
	printf("%d\n", ans);
	return 0;
}