can someone please tell me how am i suppose to start my program ?

i have 20 integers (1-20) the user will input what number should start and the count interval . its a bit confusing

only 5 numbers should be left

numbers who are already counted will not be counted again

here's an example:

user chose to start in #4 then the counting interval is 3

chosen nos are :7 10 13 16 19