You just took up a 30-year mortgage loan of $400,000 to buy your dream home, at a mortgage rate of 4.50 percent. You immediately decided that you will pay an additional amount of $400 (you prefer this to a 15-year mortgage because of its flexibility) every month to pay off the loan earlier. If you are able to do this each month, by how many years will you shorten the length of time it will take to pay off your loan?