n > 6 chu so => key = 6 chu so dang sau.
n < 6 chu so => key = them so 1 vao dang truoc.
Nếu Key > 1.000.000 Thì Key = Key -1.000.000
Nếu Key < 100.000 thì Key = Key +100.000"
Nếu key vẫn > 1.000.000 thi lay 6 so dang sau.