Problem Statement  
A farmer is planting a line of trees across the front of his house. He has four different kinds of trees he would like to plant. However, for aesthetic reasons, he does not want two of the same type of tree next to each other. Beyond that, any arrangement of trees is considered acceptable. You are given ints a, b, c, and d, indicating how many of each type of tree the farmer is going to plant. You are to return a long indicating the number of acceptable ways in which the trees can be ordered.  
Definition  
 
Notes  
  Each tree of a given type is identical to others of the same type, thus swapping the positions of two of the same type of tree does not consitute a new arrangement.  
Constraints  
  a, b, c, and d will be between 0 and 10, inclusive.  
Examples  
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