Sandwich Leave concept

I'm a beginner to these kind of logics.Sorry if I'm not clear to you.(pls comment I'll try max to make you understand) I'm trying to do a Leave Managemnt System(LMS) for my company.My company follows sandwich leave concept,which means if Mr.A took leave on Friday and Monday he is liable to 4 days leave(Includes sat&sun).I'm doing using JSP and servlets with MySQL Interaction.and bootstrap datepicker-Sandboox. Can anyone help me to figure out logic behind this? P.S- My application is almost done except this logic.