Provided are a method for performing wireless communication by a first device and a device supporting same. The method may include: receiving, from a second device, first sidelink control information (SCI) including information related to a resource reservation period, on a slot; determining a size of a selection window based on a remaining packet delay budget; obtaining a value of N by applying a ceiling function to a value obtained by dividing the size of the selection window by the resource reservation period; determining that resources are reserved by the second device on the N slots, after the slot in which the first SCI is received, which are spaced apart by a unit of the resource reservation period; and based on the determination, selecting a resource for SL communication in the selection window, wherein the N is a positive integer.