Going to grandmas house,granny likes cakes,and its her birthday,I want to give her cakes I have made,between my house and grandmas,I have to cross 7 bridges,but each bridge has a troll,before I can cross each bridge,I have to give them half of my cakes I am carrying,but they are kind trolls,they give me back a single cake,how many cakes do I have to leave home with to make sure I arrive at grandmas with 2 cakes