no of spanning tree eCa. where is e is edge and if n is vertices then a = e-n+1. now here e = r+2 and n = r . so by using this eCa we can get {(r+2)(r+1)r} / 6 . so now r = 0 is not possible so min value of r will be 1 , putting the value in r m will be 1.