How l can count my safe periods

what are the safe days in your period ( avoid pregnancy ) :
if we supposed that your period is usually 28 days , then your ovulation days are 19th-25th , other than those days are safe days and unlikely to get pregnant if sexual intercourse have been done 
how to know ovulation days and the safe days in period :
1. ovualtion time or the time ovum exist are the first 4 days of the last 16 days of the period , for example ;if your period is 28 days , and  if your first day of period was 1st/9 , your ovulation time is 12th-16th /9 
2. ovums last 48 hours in the uterus and can get fertilized any time during this period of time 
3. male semen last up to 5 days in female uterus and will be able to fertilize during this period of time , but the earlier the better 
general ways to avoid pregnancy :
1. avoid ovulation days 
2. IUD 
3. condoms 
4. oral contraceptive pills 

what are the safe days in your period ( avoid pregnancy ) :
if we supposed that your period is usually 28 days , then your ovulation days are 19th-25th , other than those days are safe days and unlikely to get pregnant if sexual intercourse have been done 
how to know ovulation days and the safe days in period :
1. ovualtion time or the time ovum exist are the first 4 days of the last 16 days of the period , for example ;if your period is 28 days , and  if your first day of period was 1st/9 , your ovulation time is 12th-16th /9 
2. ovums last 48 hours in the uterus and can get fertilized any time during this period of time 
3. male semen last up to 5 days in female uterus and will be able to fertilize during this period of time , but the earlier the better 
general ways to avoid pregnancy :
1. avoid ovulation days 
2. IUD 
3. condoms 
4. oral contraceptive pills