Given a sector with a radius of 2cm and a central angle of 120°, find the arc length.
已知某扇形半径为2cm，圆心角120°，求弧长．
<gadget id="calculator">4 / 3</gadget>
<output>4/3 = around 1.333333</output>

<result>4/3 = around 1.333333</result>