The Sharks and the Jets have decided to settle their differences with a game of Snakes and Ladders, in which movement is determined by the roll of a single die. The problem is, neither group trusts the other not to gain an advantage by swapping in a loaded die at a crucial moment. Officer Krupke, who wants to keep the peace, comes up with a novel proposal. Instead of rolling one die, each side will roll a die of its own choosing each time a die roll is required. Then the two die rolls will be combined by a simple formula that produces a number from one to six. This formula has the property that if either one of the two dice is fair, the results will be evenly distributed among the six numbers. Thus either side can guarantee a fair result just by making sure that its own die is fair. What formula might Officer Krupke have suggested?