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
1.1is a top, front, and right side perspective view of bags embodying our new design; 1.2is a front elevation view thereof; 1.3is a rear elevation view thereof; 1.4is a right side elevation view thereof; 1.5is a left side elevation view thereof; 1.6is a top plan view thereof; and 1.7is a bottom plan view thereof; this article is a bag that stores various items such as wallets, smartphones, cosmetics, accessories, etc; it is portable by using a strap connected to the main body, and is used with being attached to and detached from electric kickboards and bicycles.