Excellent explanation. Thanks!

Your code doesn't implement what Dey suggests. In order to do that with your current code (24051579), you should break the while loop when ptr1 = 1 and ptr2 = n, then add ⌊x / n⌋ to the answer.

Another formula for the same number:

Let tn be the total number of pillows, y ≤ x - 1. As Borna pointed out:

tn = (x - 1) + (x - 2) + (x - 3) + ... + (x - (y - 1)) + (x - y)

Then, if we rearrange the summands:

tn = (x + x + x + ... + x) - (1 + 2 + 3 + ... + (y - 1) + y)

I think there is an error in problem B solution: PolandBall will have an advantage of one additional word if k has an odd number of words. If k has an even number of words then each player can say the same amount of repeated words and therefore subtract the same number of words to the other player's set of words, so there is no advantage in this case.

Great contest, thanks!