You are standing on a sidewalk when a police car passes you traveling 25 m/s and sounding its siren. Behind this car, a second police car is traveling toward your position at a speed of 32 m/s. Both cars have sirens that emit identical frequencies. If you measure a beat frequency of 125 Hz in the period of time after the first car passes and before the second car reaches you, what is the frequency of the sirens on the police cars?