Need help with Hurst Exponent

Hi there =) *And sorry for my English, in advance*
I have a task to calculate hurst exponent by method of linear regression. And I have text description of solution. It looks very easy, but always i get values, that go out from range 0..1. Usually, value is 1.9 or something similar. Sometimes it gets negative value that is close to zero.
I have looked over code about thousand times but couldn't see a mistake.

var

max_z,min_z,x_m:real; //max and min of cumulative sum and mean value of X for every Tau

st,ss,sst,st2 :real;

Al, Herst: real;

x_vr:array of double;   //a piece of array with length=tau

i, j, nach: integer;

begin

//file opening and getting values of X array are in another function

nach:=3;    //initial value of tau

Setlength(ln_rs,l-nach); //length of  ln(R/S) array

Setlength(ln_t,l-nach);  //length of  ln(tau) array

Setlength(r,l-nach);   //length of  R array

Setlength(s,l-nach);   //length of S array



  //Let's start

for tau:=nach to l do  //we will change tau

  begin

  Setlength(x_vr,tau+1); //set new local array (length=tau)

  for i:=0 to length(x_vr)-1 do

  x_vr[i]:=x[i];


  x_m:=Mean(x_vr);    //mean value

  Setlength(y,tau+1);   //length of array of difference from mean value

  Setlength(z,tau+1);   //length of array of cumulative sum

  for i:=0 to tau do

    y[i]:=x_vr[i]-x_m;      //difference from mean value


  z[0]:=y[0];

  for i:=1 to tau do      //cumulative sum

    for j :=i downto 0 do

      z[i]:=z[i]+y[j];


  max_z:=z[0];

  for i:=1 to tau do        //max of cumulative sum

   max_z:=max(max_z,z[i]);


  min_z:=z[0];

  for i:=1 to tau do        //min of cumulative sum

    min_z:=min(min_z,z[i]);


  r[tau-nach]:=max_z-min_z;    //R value

  s[tau-nach]:=0;

   for i:=0 to tau do

     s[tau-nach]:=power(y[i],2)+s[tau-nach];         //S value


   s[tau-nach]:=sqrt(s[tau-nach]/(tau+1));


  //new array values

  ln_rs[tau-nach]:=Ln(R[tau-nach]/S[tau-nach]);	// ln(R/S)

  ln_t[tau-nach]:=ln(tau);						// ln (tau)


end;    //End of calculating


//Method of Least squares

 for i:=0 to length(ln_rs)-1 do	

  st:=st+ln_t[i];

 st:=(1/length(ln_rs))*st;


 for i:=0 to length(ln_rs)-1 do

  ss:=ss+ln_rs[i];

  ss:=(1/length(ln_rs))*ss;


  for i:=0 to length(ln_rs)-1 do

  sst:=sst+ln_t[i]*ln_rs[i];

  sst:=(1/length(ln_rs))*sst;


 for i:=0 to length(ln_rs)-1 do

  st2:=st2+ln_t[i]*ln_t[i];

 st2:=(1/length(ln_rs))*st2;



Herst:=(sst-st*ss)/(st2-st*st);      //coefficient of approximal function

al:=ss-st*Herst;

Thanks =)