Ruth for Ruth for 20

Rahab is something like David's great, great grandmother. So we're told at the end of Ruth in verse 20 of Ruth for Ruth for 20. Nashon fathered Salman, Salman fathered Boaz, Boaz fathered Obed, Obed fathered Jesse and Jesse fathered David. How does that work? I believe it's Salman who's connected to Ruth in Matthew chapter one,. Matthew one five, Salman the father of Boaz by Rahab.