A football player, starting from rest at the line of scrimmage, accelerates along a straight line for a time of 1.6 s. Then, during a negligible amount of time, he changes the magnitude of his acceleration to 1.8 m/s, he continues in the same direction for another 1.2 s, until he reaches a speed of 3.4 m/s. What is the value of his acceleration (assumed to be constant) during the initial 1.6 s period?