check the picture below.
make sure your calculator is in Radian mode, since the angle is in radians.
in case you're wondering, in the Unit Circle, the radius is 1, and due to that, the arc's length, in this case an arc of 2 units, is the same value as the angle it makes in radians, so, if the arc is 2, then the angle it makes, in this case clockwise, in the direction as a clock moves, or a negative angle of -2 radians then.
