Poisson Distribution Help

Ok, here's the problem.

The Number of medical emergency calls per hour has a Poisson distribution with parameter $\displaystyle lambda$. A record of emergency calls is available for a sufficient amount of time and parameter $\displaystyle lambda$ is assumed to be the same throughout the available recording of calls. Number of emergency calls at different hours are independant. If $\displaystyle lambda$ equals 1, what is the probability of at least 20 emergency calls in the 10 consecutive hours of a single medical response team shift??

I think I must be using the wrong formula, or just approaching this problem incorrectly. My probablities keep coming out in varieties of 1.9 to the negative 60th, etc. I really don't think that answer is correct.

Help please?