# Why is this attack complexity equal that exact number of bit operations?

in the this paper,section 3,autors attack hamsi-256. Im trying to make a parametrized version, so i need to understand how do they estimate the complexity of attack in bit operations,that reads as followed Can anybody say why they dont count number of operations to generate a tree with $$2^{224}$$ leaves,and why is the summ looks like that? I know im asking a hard question, but i have noone to explaing such thing to me.