dung roi mien phi that. Chi can ngoi xem quang cao cua no la duoc tien roi.
Moi lan gioi thieu cho 1 nguoi thi duoc 1k. 10nguoi thi 10k. Roi moi nguoi ban gioi thieu moi them 10 nguoi nua, roi 10 nguoi nua gioi thieu 10 nguoi nua. => tien cua ban se duoc nhân lên theo so nguoi. Ban minh test roi.