Is it safe cum inside a 5 days girl after her menstration period?

Not relevant. When a woman is on her period there is no increase or decreased risk regarding sexual activity. Other than risk of sexually transmitted disease and pregnancy, there is nothing dangerous about ejaculation and menstrual bleeding. Many women do not want to have intercourse while they are bleeding but it is not more dangerous.
Safe from what. Pregnancy has occurred from unprotected sex during any day of a woman's cycle. There are what is considered more likely days to conceive, but any day is just fine. Menses just reflects left over stuff from last month. The new month ahead may have some surprises. If you want to avoid pregnancy, use a condom before any exposure.