In the xy-plane, the region y > 0 has a uniform magnetic field B1k^ and the region y < 0 has another  uniform magnetic field B2k^ . A positively charged particle is projected from the origin along the positive y-axis with speed v0=πm/s at  t=0, as shown in the figure. Neglect gravity in this problem. Let t=T be the time when the particle crosses the x-axis from below for the first time. If  B2=4B1, the average speed of the particle, in  m/s, along the x-axis in the time interval T is___________.