In A Large Bowl, Combine The Popcorn, Cereal, Pretzels And 1/2 Cup M&M'S. In A Microwave, Melt Marshmallows And Butter; Stir Until Smooth. Fold Into Cereal Mixture. Spread Into A Greased 11-In. X 7-In. Dish. Sprinkle With Remaining M&M'S; Press Lightly. Cut Into Bars.