The ornamental design for a bag, as shown and described.
1. Bag
1.1: Perspective
1.2: Front
1.3: Back
1.4: Right
1.5: Left
1.6: Top
1.7: Bottom
The bag comprises a body which, when viewed from the front, has a substantially trapezoidal shape, with a bottom whose ends have a rounded contour and are curved upwards, so that said bottom resembles the board of a skateboard. The top of the body has a zipper that also extends down the sides of the body; in the reproductions of the design, the blurred elements in light gray (handles and shoulder strap) are not claimed.