Hansel and Gretel, a married couple, manage apartments and they are required to live in the managers' apartment as a condition of their employment. Instead of providing the apartment to Hansel and Gretel rent-free, the owner of the apartment building gives Hansel and Gretel a housing allowance of $600, which they use to pay rent on the managers' apartment. Hansel and Gretel pay $600 per month in rent. If they did not live in the managers' apartment, Hansel and Gretel could live in another apartment building where they would only pay $500 in rent. What amount, if any, must be included in Hansel and Gretel's gross income?