program ChuViHinhVuong;

var
  canh: integer;
  
begin
  repeat
    writeln('Nhap do dai canh: ');
    readln(canh);
    if canh <= 0 then
      writeln('Do dai canh khong hop le. Nhap lai!');
  until canh > 0;

  writeln('Chu vi hinh vuong la: ', canh * 4);
  readln;
end.