uses crt;
var n,a,c,t:longint;
begin
    clrscr;
    write('n=');
    readln(n);
t:=0;c:=n;
while c<>0 do
   begin
       a:=c mod 10;
       t:=t+a;
       c:=c div 10;
   end;
writeln('tong chu so:',t);
readln
end.

ok

TH1: $n\in\mathbb N$

uses crt;
var n:word;

function solve(n:word):longword;
begin
    if(n<10) then exit(n) else exit(n mod 10+solve(n div 10));
end;

begin
clrscr;
    readln(n);

    //Recursive sum digits and print result
    write(solve(n));
readln;
end.

TH2: $n\in\mathbb Z$

uses crt;
var n:longint;

function solve(n:longint):longint;
begin
    if(n<10) then exit(n) else exit(n mod 10+solve(n div 10));
end;

begin
clrscr;
    readln(n);
    
    //Delete char '-'
    if(n<0) then n:=-n else n:=n;

    //Recursive sum digits and print result
    write(solve(n));
readln;
end.