Robert is repaying a debt with 21 annual payments of 900 dollars each, the first coming a year from now. At the end of the 4th year, he makes an extra payment of 1800 dollars. He then shortens his remaining payment period by 2 years, and makes level payments over the remaining time. If the effective rate of interest is 7.7 percent, how large is his new annual payment?
Answer = dollars.