program SumOfDigits;

var
number, sum, digit: integer;

begin
sum := 0;

write('Enter a four-digit number: ');
readln(number);

if (number < 1000) or (number > 9999) then
begin
writeln('Please enter a valid four-digit number.');
end
else
begin
digit := number mod 10;
sum := sum + digit;
number := number div 10;

digit := number mod 10;
sum := sum + digit;
number := number div 10;
digit := number mod 10;
sum := sum + digit;
number := number div 10;
digit := number mod 10;
sum := sum + digit;
writeln('Sum of digits: ', sum);

end;
end.