## proof that there exists a smallest positive linear combination?

A theorem from number theory states that, if a and b are nonzero integers, then there exists a smallest positive linear combination of a and b.

This is my proof:

Let S be a set such that S = {w$\in$Natural numbers : w=am+bn} , where a and b are positive integers, m and n are any integers, and w is by definition a linear combination of a and b.
Suppose S is nonempty. Then S is a subset of the natural numbers. Then by the Well Ordering Principle, S has a smallest (positive) element. Thus there exists a smallest positive linear combination of a and b.

Is this correct? Or am I missing something? My professor said that fastest way to prove this is by contradiction, but it seems to me that just directly proving by the well ordering principle is faster?

 PhysOrg.com science news on PhysOrg.com >> Hong Kong launches first electric taxis>> Morocco to harness the wind in energy hunt>> Galaxy's Ring of Fire
 You really don't have to assume that S is non-empty, as you can easily prove this fact.