CREATE TABLE table_13642 ("Stage" TEXT, "Winner" TEXT, "General classification" TEXT, "Points classification" TEXT, "Mountains classification" TEXT, "Young rider classification" TEXT)

What's the mountains classification of winner Carlos Sastre when the general classification is Denis Menchov?
SELECT "Mountains classification" FROM table_13642 WHERE "General classification" = 'denis menchov' AND "Winner" = 'carlos sastre'
SELECT "Mountains classification" FROM "table_13642" WHERE "General classification" = 'denis menchov' AND "Winner" = 'carlos sastre'