Sean, who is single, received social security benefits of $8,640, dividend income of $12,660, and interest income of $2,170. Except as noted, those income items are reasonably consistent from year to year. At the end of 2018, Sean is considering selling stock that would result in an immediate gain of $10,340, a reduction in future dividends of $1,085, and an increase in future interest income of $1,585. What amount of social security benefits is taxable to Sean?