At a restaurant, the density function for the time a customer has to wait before being seated is given by $$f(t) = \begin{cases} 0 & \text{if} \;\; t<0\\ 1 e^{-1 t} & \text{if} \;\; t\geq 0. \end{cases}$$ Find the probability that a customer will have to wait at least $6$ minutes for a table.