For a random walk of 10 000 steps, what is the chance of ending up exactly where we started? (10 000 choose 5000)/2^10000, right? This calculation can't be done on a calculator, so I use Stirling's formula: ln M! = M lnM - M + 0.5 ln (2*pi*M), knowing that (10 000 choose 5000) = 10 000!/(5000! * 5000!) But I also need a way to approximate 2*10 000. How do I do that?