When you type a number like 2200 into a sketch, the compiler assumes this is an int. Unfortunately, 2200×6550 is too large to fit in an int (the max is around 32000, or around 65000 if unsigned). If this sketch was running on a PC, you might get an"overflow" error at this point, but not on an Arduino. The error is not detected and a corrupted result is used for the rest of the calculation.
The "L" tells the compiler that this number is a long rather than an int. A long can hold a huge number, so no overflow and the correct answer.
For further improvement, remember what I said about measuring pulses in pairs.