Greatest $n$ that can be written in the form of $ax+by=n$

In a diophantine equation $ax + by = n$ with $(a, b) = 1$, the greatest possible value of $n$ such that both $(x, y)$ are not positive is $ab − b − a$?

This is given in my module (without any proof). I am assuming that "both $(x, y)$ are not positive" means at-least one must be negative. I was wondering how to prove this.

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I suppose that the question is meant to be about $n$ that cannot be written as $ax+by$ with $x,y\geq0$. If you confirm, please change your title, which is nonsensical as it stands. –  Marc van Leeuwen Apr 2 '12 at 8:02
The statement of the problem is not clear. Among other things, are you looking for non-negative solutions or positive solutions? Also, "both $x$ and $y$ are not positive" could very well mean that each of $x$ and $y$ is $\le 0$. With rewording, you will need (i) the argument of Gerry Myerson that $ab-b-a$ is not representable with both $x$ and $y \ge 0$ and (ii) everything bigger is representable. For that, again the basic tool is the formula for the general solution of the Diophantine equation given a particular solution $(u,v)$. –  André Nicolas Apr 2 '12 at 11:40
First note that if $x=b-1$ and $y=-1$ then $ax+by=ab-b-a$.
Then what you have to know is that if $(u,v)$ is one solution to $ax+by=n$ then the full set of solutions is given by $x=u+kb$, $y=v-ka$ where $k$ runs through the integers (positive and negative (and zero)).
So to increase $y$, you have to decrease $x$ by $b$ (or more), so you can't get a positive solution when $n=ab-b-a$.
By the way, I'm assuming that $a,b$ are meant to be positive integers.