746. Lockdown in Mirzapur
The nationwide lockdown due to COVID-19 has also affected Mirzapur. Guddu Bhaiya and Bablu Bhaiya are adhering to the lockdown rules and practicing social distancing. However, they are getting bored at home and have come up with a list of activities to keep themselves occupied, such as counting kattas and planning barfi transport routes. They want to know how many ways they can choose a specific number of activities to do each day.
1 <= R <= N <= 20