Xử lí số nguyên lớn có dấu

https://vfo.vn/t/showthread.php?96739-baI-TAP-XU-LY-SO-NGUYEN-LON-MOI-NGUOI-GIUP-VOI

thủ tục cộng của mình

procedure doc;
var
    x:char; I,J,so,code:integer;
begin
     assign(f,fi);reset(f);
     readln(f,n,m);
     for i:=1 to MAX do begin a[i]:=0;b[i]:=0;end;
     j:=n;
    for i:=1 to n do
     begin
     read(f,x);
     val(x,so,code);
     a[j]:=a[j]+so;
     j:=j-1;
     end;
     readln(f);
     j:=m;
     for i:=1 to m do
     begin
     read(f,x);
     val(x,so,code);
     b[j]:=b[j]+so;
     j:=j-1;
     end;
     if m>n then d:=m else d:=n;
     close(f);
end;
procedure xl;
var i:INTEGER;
    S,V:BYTE;
begin
     s:=0;
     v:=0;
     for i:=1 to D+1 do
     begin
     s:=a[i]+b[i]+v;
     b[i]:=0;
     b[i]:=s mod 10;
     v:=s div 10;
     end;
end;
procedure xuat;
var i,dem:integer;
begin
     assign(f,fo);rewrite(f);
     DEM:=0;
     for i:=D+1 downto 1 do if b[i]<>0 then begin dem:=i;break;end;
     writeln(f,DEM);
     for i:=dem downto 1 do write(f,b[i]);
 
     close(f);
end;
BEGIN
     SINH;
     doc;
     xl;
     xuat;

const fi ='hieu.inp';
      fo ='hieu.out';
       max= 32001;
 type mmc = array[0..max] of byte;
 var m,n: integer;
 a,b:mmc;
   procedure nhap;
     var code, t, j, i: integer;
          f: text;
             x: char;
   begin
     assign(f,fi);
     reset(f);
     readln(f,m,n);
         for i:= 1 to max do
           begin
              a[i]:= 0; b[i]:= 0;
              end;
                  j:=m;
             for i:= 1 to m do
               begin
                    read(f,x);
                    val(x,t,code);
                       a[j]:=a[j]+t;
                     dec(j);
                     end;
                     readln(f);
                      j:=n;
                       for i:= 1 to n do
                         begin
                           read(f,x);
                             val(x,t,code);
                             b[j]:= b[j]+t;
                             dec(j);
                           end;
       close(f) ;
      end;
 function SOSANH(VAR m: integer; VAR a:mmc;VAR n: integer;var b:mmc ) : integer;
 var i : integer;
begin
    if (m > n) then exit;  SOSANH:=-1;
    if (m < n) then  exit ; SOSANH:= 1;
    for i := n downto 1 do
        begin
            if (a[i] > b[i]) then exit;  SOSANH:=-1;
            if (a[i] < b[i]) then exit; SOSANH:=1;
        end;
      exit;
     SOSANH:=0;
end;
   procedure tru(VAR a:mmc; VAR m: integer; VAR b: mmc ;VAR n: integer);
     var i,j: integer;
        nho:byte;
       begin


       nho:= 0;
        j:=1;
            for i:= 1 to m do
             begin
                if a[i] >= b[i] then
                  begin
                  a[i]:= a[i] -b[i] - nho;
                 nho:=0;
                 end else begin
                   a[i]:= a[i]+10 - b[i] -nho;
                   nho:=1 ;
                   end;
                        if a[i]> 0  then
                        j:=i;
                   end;
                    m:=j ;
            end;
 procedure xuat;
     var f: text;
         i: integer;
         t:integer;
         begin
          assign(f,fo);
           rewrite(f);
            t:=sosanh(m,a,n,b);
              if (t>0) then
                begin
                  tru(a,m,b,n);
                      for i:= m downto 1 do
                         write(f,a[i]);
                     end else
                     if (t<0 ) then
                      begin
                         tru(b,n,a,m);
                         write(f,'-');
                          for i := n downto  1 do
                             write(f,b[i]);
                         end else
                           if (t = 0) then
                           write(f,0);
                           close(f);
                 end;
begin
  nhap;
  xuat;
end.