where s is the score of the node, n is the number of times the node has been visited, c is the search coefficient (c = 1.5 in our case), and N is the number of times the parent node has been visited. Equation ( ) corresponds to Equation of the Upper Confidence Bound 1 (UCB1) , which is well known in reinforcement learning. At this time, the depth is increased by one with the selected node.