The solution idea comes from dearrangement. At a time from first m integer you have to dearrange (m-k) integer. And also from (m+1)'th to n'th integer you can dearrange them or not. So iterate over (n-m) and for every time suppose i you keep them at their position, so you have (n-k-i) integer which you can dearrange. Add those ways in every iteration. And at last multiply with mCk