An 20-year annuity pays $500 a year at year end. If the annuity's first cash flow starts at the end of year 1 and the interest rate is 4%, what is

An 20-year annuity pays $500 a year at year end. If the annuity's first cash flow starts at the end of year 1 and the interest rate is 4%, what is the present value of this annuity? 

Please include formula and step by step calculations