NWAYS - Editorial

I could not understand how ∑i=k+1n+k(ik+1) came from ∑i=k+1n+k(ik+1). Could you please elaborate?

Hi i still couldn’t understand how is it ∑i=1 n(n−i+k+1 k+1) = ∑i=k+1 n+k(i k+1)
It will be good help if someone breaks down them into sub steps?
I went through link posted by amitkpandeykgp

Its just a change of variable. Just expand both the expressions and you will see that both of them expand to same expression