How to count the number of all elements of an array in the entire span, and not just in 3 places?

Question

Lazarus

How to count the number of all elements of an array in the entire span, and not just in 3 places? (Number - k)
Thanks in advance

procedure TForm1.Button1Click(Sender: TObject);
var
  x:array [1..20] of real;
  i,n,k: integer ;
  s,a,b:real;
begin
 n:=strtoint(edit1.text);
  stringgrid1.ColCount:=n;
  for i:=1 to n do
  x[i]:=strtofloat (stringgrid1.Cells[i-1,0]);
  a:=strtofloat (edit2.text);
  b:=strtofloat (edit3.text);
  s:=0 ; k:=0; i:=3;
   while i<=n do
  begin
   if(x[i]>=a) or (x[i]<=b) then
   begin
   k:=k+1;
   s:=s+x[i];
   i:=i+3;
   end;
   label5.caption:='Сумма элемнтов массива на 3-их местах принадлежащих промежутку равно ' + floattostr(s);
   label4.caption:='Количество эллементов массива принадлежащих промежутку равно ' + floattostr(k);
end;
end;