#include<stdio.h>
char s[100000];

int main(){


    scanf("%s", s);
    int len = strlen(s),i,hugs=0,ans;
    for(i=0;i<len;i++ ){
    s[i]!=s[len-i];
        hugs=hugs+1;}
        ans=hugs/2;
        printf("%d\n",ans);
        return 0;

}
