4 factorial is equal to 4 times 3 times 2 times 1. It's actually a fun thing to play with. Factorials get large very, very, very, very fast. Anyway, this denominator right over here can be rewritten as k factorial. This right over here can be rewritten as k factorial. Is there any way to rewrite this numerator in terms of factorials? If we were to write n factorial, let me see how we can write this.