You receive a 4-year $18,000 negative amortization loan with an interest rate of 10% p.a., to be repaid in four annual installments. The loan requires that you make total payments of $300 at t = 1, $500 at t = 2, and $400 at t = 3, with the remaining loan balance paid at maturity. What is the total payment amount at t = 4, rounded to the nearest dollar?