Doppler Effect Police Car Question

A police car emitting a siren wail (400 Hz) is driving towards a fleeing suspect at v/35, where v is the speed of sound. The suspect is running away at v/55. What frequency does the suspect hear?

For this problem, I used the Doppler Effect equation, and got 419.361 as my answer. I used + on the top and - on the bottom. Do I need to switch the two?