I’ve constructed a sequence thusly:

I’ve started with the natural numbers 1,2,3,4,5,6,7,8,9,10 etc.

I’ve kept the first number as it is, then reversed the order of the next two, then reversed the order of the subsequent three, then reversed the order of the next four etc:

1,3,2,6,5,4,10,9,8,7,15,14,13,12,11,21,etc

Your task is to construct a formula that will instantly give you the nth term in this crazy sequence.