No. of node in binary with 1 degree i.e. leaf node
2 degree i.e. only root note
node with 3 degree i.e. internal node except root (degree 2)
so number of degree 3 vertices are calculated = interms of internal node -1
internal node = leaf node -1 i.e. n1-1
since from internal node root is 2 degree vertex so removes it
So no. of 3 degree vertices are = n1-1-1= n1-2