CREATE TABLE table_13640 ("Stage" TEXT, "Winner" TEXT, "General classification" TEXT, "Points classification" TEXT, "Mountains classification" TEXT, "Young rider classification" TEXT)

What is the stage of winner Carlos Sastre when the general classification was Denis Menchov?
SELECT "Stage" FROM table_13640 WHERE "General classification" = 'denis menchov' AND "Winner" = 'carlos sastre'
SELECT "Stage" FROM "table_13640" WHERE "General classification" = 'denis menchov' AND "Winner" = 'carlos sastre'