## Poisson Process

I don't understand how the poisson process has the property:

P(N(t+h)-N(t)=1) = rh+o(h) where r is the rate

because when I try to work it out i get:

P(N(t+h)-N(t)=1) = P(N(h)=1) = rh(1-rh+o(h))