[pascal] plus a big number (cộng số cực lớn trong pascal)

rogram plus_string_advance;
uses crt;
type
     int        =       integer;
     arr        =       array[1..100] of int;

var  a,b,c      :       string;
procedure inp;
var i:int;
begin
     writeln('a>=b');
     write('nhap so a: '); readln(a);
     write('nhap so b: '); readln(b);
     for i:=1 to length(a)-length(b) do
     b:='0'+b;
end;

procedure pro(a,b:string);
var d,i,j,k:int;
    code:int;
    i1,i2,kt:int;
    s       :string;
begin
     c:='  ';
     d:=length(a);
     j:=length(a);
     k:=length(a);  kt:=0;
     repeat
           val(a[j],i1,code);
           val(b[k],i2,code);
           {write('i1: ',i1,' i2: ',i2,' ');}
           if code=0 then
           begin
              i:=i1+i2+kt; {writeln(i);}
              if i>=10 then begin
                                 i:=i mod 10;
                                 kt:=1;
                                 str(i,s);
                                 insert(s,c,length(c)+1);
                            end
                            else
                            begin
                                 kt:=0;
                                 str(i,s);
                                 insert(s,c,length(c)+1);
                            end;
           end;
           d:=d-1;
           j:=j-1;
           k:=k-1;
     until d=0;
     if kt=1 then c:=c+'1';
     write('c la: ');
     for i:= length(c) downto 1 do write(c[i]);
end;

begin
     clrscr;
     inp;
     pro(a,b);
     readln;
end.