Rashi on Deuteronomy 2:1/רש"י על דברים ב׳:א׳

1ונפן ונסע המדברה THEN WE TURNED AND JOURNEYED INTO THE WILDERNESS — If they had not sinned, they would have passed by the way of Mount Seir to enter the Land from its south to its north, but because they became degenerate they had to turn towards the wilderness which is between the Red Sea and the south of Mount Seir, and they proceeded along its southern side from the west to the east, דרך ים סוף BY THE ROUTE FROM THE RED SEA — i.e. by the route which they had taken on leaving Egypt which is at the south-west corner. From there (where they were at that moment) they went towards the east.

2ונסב את הר שעיר AND WE COMPASSED MOUNT SEIR, the whole of the south side as far as the land of Moab.