CREATE TABLE table_13641 ("Stage" TEXT, "Winner" TEXT, "General classification" TEXT, "Points classification" TEXT, "Mountains classification" TEXT, "Young rider classification" TEXT)

What's the points classification of Stage 18 when the general classification was Denis Menchov?
SELECT "Points classification" FROM table_13641 WHERE "General classification" = 'denis menchov' AND "Stage" = '18'
SELECT "Points classification" FROM "table_13641" WHERE "General classification" = 'denis menchov' AND "Stage" = '18'