Two sources of sound are moving in opposite directions with velocities V1 and V2 (V1 > V2. Both are moving away from a stationary observer. The frequency of both the sources is 1700 Hz. If velocity of sound in air is 340 m/s and both V1 and V2 are much small compared to velocity of sound, then the value of V1-V2 for beat frequency to be 10 Hz as heard by observer is