Simba Travel Agency arranges trips for climbing Mount Kilimanjaro. For each trip, they charge an initial fee of $100 dollars, in addition to a constant fee for each vertical meter climbed. For instance, the price for climbing to the Shira Volcanic Cone, which is 3000 meters above the base of the mountain, is $400 dollars. Let F(d) denote the fee of a trip F (measured in dollars) as a function of the vertical distance climbed d (measured in meters). Write the function's formula.