Can anyone help me solve this - I get negative values...

When I go swimming, the distance in meters that I can swim before getting cramp in one of my hands is exponentially distributed with mean 332.4 meters. What is the probability that, on a random occasion when I go swimming, I swim a total of at most 493.1 meters without getting hand cramp given that I swim at least 324.5 meters without getting cramp?

Thanks, F