For N(E)=Aexp(-E/kT), I know that N(E) is the no. of particles with a certain energy E, but why does integrating N(E) from 0 to infinity equal to 1? Although I realize that it means that there is 100% probability to find a particle in this range, I want to know why summing up all no. of particles at all energy levels leading to probability, i.e. 1. Do I have some misunderstanding about this equation?