Multi-Step Binomial Tree

Suppose that a binomial tree has n steps and the stock has initial price S0 and then at each step, its price can only move up by a factor u or down by a factor d. Let Sk, k=0,1,2...n be the price of the stock at the end of the kth step. For n =3, calculate the fair price for the option at current time corresponding to the initial node of the tree (please write out the explicit formula)