# Placing reservations on boxes when using the Pigeonhole Principle

I am choosing $m+1$ integers from the set $\lbrace 1,2,\ldots 2m\rbrace$, and I want to use the Pigeonhole Principle to show that two of the numbers chosen differ by $1$. I want to know if my strategy (below) is accepted so that I may turn to it in more complicated problems. If it is not, please state why. I am not looking for a solution to this problem (and I can solve it in a way that doesn't use the construction below), so please do not solve it.

My Solution: We have $m$ boxes, and we reserve the 1st box for 1 and 2, the 2nd box for 3 and 4, ..., and the mth box for $2m-1,2m$. Since we're distributing $m+1$ objects into $m$ boxes, by the Pigeonhole Principle, there is a box which has two elements. By construction, the difference of these two elements is 1.

Q: Am I allowed to place reservations on my boxes?

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I’d turn it around and speak of choosing $m+1$ numbers from those same $m$ boxes and therefore having to choose two from the same box, but yes, that argument is fine. –  Brian M. Scott Feb 5 '13 at 5:09

The arguement to me looks ok, because if you select $m+1$ objects from $m$ boxes you must end up picking two from atleast one box.