Kona's apartment is located 15 miles away from his office. On Monday, he drove from his apartment to the office. After work, he drove 9 miles to the grocery store before heading back to his apartment, which is 6 miles from the grocery store. On Tuesday, Kona drove directly from his apartment to the office and back without any stops. How many additional miles did Kona drive on Monday compared to Tuesday? Let's think step by step and output the final answer within \boxed{}.
user