Calculating days between days by individual person

Hi. One last question for the day!

Spreadsheet attached. I need to calculate the number of days from a hospital discharge to the next admission. These are in two different columns. One person an have 1 or many admissions. If they don't have a readmission, I just want an NA returned. The values in the attached example spreadsheet were calculated by hand as an example of what I'm need.

Thanks!