Peppery Scalloped Potatoes

1 can (10-3/4 ounces) condensed cream of mushroom soup, undiluted
1 ½ cups milk
1/2 to 1 teaspoon salt
1/8 teaspoon cayenne pepper
5 cups thinly sliced peeled potatoes
1/4 cup butter or margarine, melted
1/4 cup all-purpose flour

In a small bowl, combine the soup, milk, salt and cayenne; set aside. Place a third of the potatoes in a greased 13-in. x 9-in. x 2-in. baking dish; layer with a third of the butter, flour and soup mixture. Repeat layers twice. Bake, uncovered, at 350 for 1 hour and 20 minutes or until the potatoes are tender.