we need to add all the nodes at every level (Assuming Level starts from 1 )
Every nodes will have 4 block pointer (Max) So at level 6 we will have (4*4*4*4*4 ) + for level 5(4*4*4*4) + for level 4(4*4*4) + for level 3(4*4) + for level 2(4) + for level 1(1)