Prime Distance On Tree

You are given a tree. If we select 2 distinct nodes uniformly at random, what's the probability that the distance between these 2 nodes is a prime number? ### Input The first line contains a number **N**: the number of nodes in this tree. The following **N**-1 lines contain pairs a[i] and b[i], which means there is an edge with length 1 between a[i] and b[i]. ### Output Output a real number deno