For 2018, Randy, an unmarried taxpayer, has wages of $45,000. He has a significant amount of income from dividends and interest and therefore expects to have a federal income tax liability of $18,000. His total federal income tax withholding for the year is $9,500. Randy had $100,000 of adjusted gross income and a federal tax liability of $11,000 on his 2017 return. To avoid a penalty for underpayment of estimated taxes, what is the minimum amount of total estimated tax that Randy must pay in 2018? Assume the payments are made on a timely basis.