How are proofs formatted when the answer is a counterexample? Suppose it is asked:

Prove or find a counterexample: the sum of two integers is odd

The fact that 1 + 1 = 2 is a counterexample that disproves that statement. What is the proper format in which to write this? I will provide my attempt.

Theorem: the sum of two integers is odd
Proof:
1 + 1 = 2
=> the sum of two integers may be even

I understand the math, but I'm not sure how to properly present a counterexample relative to an initial theorem. Was this an acceptable presentation of a proof?

I think you should add some text to help the reader. Here's my interpretation of an acceptable presentation:

Suppose that for any two integers $a$ and $b$, $a+b$ is odd. Letting
  $a=b=1$, it follows that $a+b=1+1=2$ is odd, a contradiction.

Edit: As pointed out in the comments, your presentation is not acceptable, since you have written "Theorem: the sum of two integers is odd". What you should have written is "Theorem: there exists two integers whose sum is not odd" (the negation of the statement).
