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.

uses crt;

var n,s: longint;

begin
clrscr;
repeat

clrscr;

write('Nhap 1 so co 4 chu so: ');   readln(n);
if (n<1000) or (n>9999) then write ('dieu kien so nay phai co 4 chu so');
until (n>=1000) and (n<=9999);
while n>0 do
begin
s:=s+n mod 10;
n:=n div 10;
end;
write('Tong cac chu so la: ',s);
readln;
end.