Poisson Distribution Help

Ok, here's the problem.

The Number of medical emergency calls per hour has a Poisson distribution with parameter . A record of emergency calls is available for a sufficient amount of time and parameter is assumed to be the same throughout the available recording of calls. Number of emergency calls at different hours are independant. If 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?