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Hi! I am an ACMer from CSU. This contest made by me is to celebrate my girlfriend’s birthday although the problems in this contest do not relate to her in fact. :) Any way, happy birthday to you, honey!
Thanks to LCY, I have this chance to share my ideas and works with you. Good luck and have fun!
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We have a graph with size = N like that in Figure 1. Then we are going to find a downward path from the top node to one bottom node.
First, we select the top node as the beginning. Then at any node, we can go horizontally or downward along the blue edge and reach the next node. The finding will be end when we reach one of the bottom nodes. After that we can get a downward path from the top node to one bottom node. Note that we can not pass a blue edge that we have passed ago during each finding.
Your task is to calculate there exists how many downward paths.