Program tinh_tong;
uses crt;
Var i,n,sum: integer;
Begin
write('Nhap so n: n= '); readln(n);
while (n>= 20) or (n<=3) do begin ('Nhap lai n: n='); readln (n); end;
sum:=0;
for i:=1 to n do sum:=sum+i;
writeln('Tong can tinh là:', sum);
Readln;
End.