Suggest Dose Of Mifepristone To Prevent Pregnancy

i am a girl of 24years married and i do not want pregnancy so soon we are doing sex without using any contraceptive device . using only safe period time but at time in unsafe period also due to my husband request . tell me how many times i can take mifipriston in a month and how much gap must be there ,what should be the dose after unprotected sex .and is there any effect on my periods after taking the drug