Найти произведение больших чисел

uses crt;
 
var rez,a,b:real;
 
 
begin
    Write('Ââåäèòå 1-å íàòóðàëüíîå ÷èñëî: ');readln(a);
    Write('Ââåäèòå 2-å íàòóðàëüíîå ÷èñëî: ');readln(b);
    if (a>5E9) and (a<5E12) and (b>5E9) and (b<5E12) then
    begin
    rez:=a*b;
    Writeln('a*b = ',rez:3);
    end
    else
    Writeln('Íåâåðíîå íàòóðàëüíî ÷èñëî ');
end.

uses crt;
label m1,m2;
var a,b:string;
i,j,k,m:integer; c,d,x:array[1..253] of integer;
begin
clrscr;
m1:begin writeln('ââåäèòå ïåðâîå ÷èñëî');
readln(a);
m2:
if (length(a)>10) and ((a[1]='5') or (a[1]='6') or (a[1]='7') or (a[1]='8') or (a[1]='9')) and(length(a)<13) then
writeln('ââåäèòå âòîðîå ÷èñëî')
else begin writeln('îøèáêà ââîäà ïåðâîãî ÷èñëà');
goto m1;
end;
readln(b);
if (length(b)>10) and ((b[1]='5') or (b[1]='6') or (b[1]='7') or (b[1]='8') or (b[1]='9')) and(length(b)<13) then
j:=length(a)
else begin writeln('îøèáêà ââîäà âòîðîãî ÷èñëà');
goto m2;
end;
for i:=1 to length(a) do
begin
val(a[i],c[j],m); j:=j-1;
end;
j:=length(b);
for i:=1 to length(b) do
begin
val(b[i],d[j],m); j:=j-1;
end;
if length(a)<length(b) then k:=length(b) else k:=length(a);
k:=k+1;
for i:=1 to k do
x[i]:=0;
for i:=1 to k do
begin
x[i]:=c[i]+d[i]+x[i];
if x[i]>=10 then
begin
x[i+1]:=x[i+1]+1;
x[i]:=x[i] mod 10;
end;
end;
if x[k]=0 then k:=k-1;
for i:=k downto 1 do write(x[i]);
end;
end.