Hàm nhân 2 số lớn pascal

Program so_lon;
 var
  st,st1,st2:string;
 Function Cong(s1,s2:String):String;
Var
    L1,L2,Max,i,tam,a,b,code,nho:Integer;
    h,h1:String;
Begin
 L1:=length(s1);
 L2:=length(s2);
if L1>L2 Then Max:=L1 Else Max:=L2;
 For i:=L1+1 to Max do s1:='0'+s1;
For i:=L2+1 to Max do s2:='0'+s2;
nho:=0; h:='';
For i:=Max downto 1 do
   Begin
       val(s1[i],a,code);
       val(s2[i],b,code);
       tam:=a+b+nho;
if tam>=10 Then nho:=1  Else nho:=0;
  str(tam Mod 10,h1);
h:=h1+h; End;
if nho=1 Then h:='1'+h;
cong:=h;
 End;
 Begin
  write('nhap so thu nhat'); readln(st1);
  write('nhap so thu hai'); readln(st2);
  writeln('ket qua la', cong(st1,st2));
  readln;
 End.