program so4chuso;
uses crt;
var n:longint;
s:integer;
begin
clrscr;
repeat
write('Nhap so: '); readln(n);
if (n<1000) or (n>9999) then writeln('So nay phai co 4 chu so. Nhap lai!');
until (n>=1000) and (n<=9999);
s:=0;
while n>0 do
begin
s:=s+n mod 10;
n:=n div 10;
end;
write('Tong cac chu so cua so nay la: ',s);
readln
end.