We just want the first k terms of it. What if we divide this by n minus k factorial? Let's think about what that is going to do. If we have n minus k factorial, that is the same thing as n minus k times n minus k minus 1 times n minus k times n minus k minus 2, all the way down to 1. When you divide these, the 1's are going to cancel out. What you may or may not realize, and you can work out the math, is everything is going to cancel out here until you're just left with up here, everything from n times n minus 1 to n minus k minus 1. If you expand this out or if you distribute this negative number, this is the same thing as n minus k plus 1. n minus k plus 1 is the integer that's 1 larger than this right over here.