William owns a four-unit apartment building for which he receives $500 per month per unit. Three of the units were rented for the entire 12 months. The fourth unit was occupied from January 1 to April 30, 2023. Upon vacating the unit the tenant was not refunded his security deposit of $500 due to damage to the unit. The unit was subsequently rented for 1 year beginning August 1, 2023. On August 1, 2023, the new tenant paid the first and last months' rent and a refundable security deposit of $500. What is William's rental income for 2023