In 2021, Marvin, a single, cash basis taxpayer, took a $2,400 itemized deduction for state income taxes paid. The deduction was not limited by the SALT cap. His total itemized deductions in 2021 were $17,800. In 2022, Marvin received a $300 refund when he filed his 2021 state income tax return. How much of his refund must Marvin include in his gross income for 2022?