Cумма чисел

Program primer;
uses crt;
var a:longint;
S:integer;
begin
clrscr;
S:=0;
Write('a='); Readln(a);
While a>0 do
begin
S:=S+a mod 10;
a:=a div 10;
end;
Writeln('S',S);
Readkey;
end.