#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

#define rd_size() ({long s[18];fstat(0,&s);s[6];})
#define rd_init() char*rp=({char*mmap();mmap(0l,1l<<25,1,2,0,0ll);})
#define wt(v) ({unsigned _z=v;do*--wp=_z%10+48;while(_z/=10);})
#define wt1(v) ({char wbuf[64],*wp=wbuf+sizeof wbuf;wt(v);write(1,wp,wbuf+sizeof wbuf-wp);})

int main(){
	int n=rd_size()-1;
	int i=0;
	int z=0;
	rd_init();
	while(i<n){
		z+=rp[i++]!=rp[--n];
	}
	wt1(z);
	_exit(0);
}