Problem Description, Input and Output:
Ok, this is one of the problems that just a few people have solved, and my intension is not to raise that number, is more like to explain how can it be solved and show a path and with that all of you are going to be able to solve it by yourselves.
Well, with all the restrictions and things described in the description of the problem we can assume a lot of things, and that make this problem easier to solve, actually theres just 1 case that is the hardest to solve, the rest is easy.
One way of solving this problem (the one i took) would be considering the graph as a hierarchical tree, each child has one more grade than the father (father => grade 1 ; son => grade 2). and that make it easier to verify the restrictions.
source code