Problem Statement  
We want to buy a rectangular plot on a rectangular vegetable field. We want our plot to be as big as possible, but we only have the resources to grow a maximum of 2 different kinds of vegetables on it. You are given a String[] field, where each character represents a square in the field. All squares are of equal size. The jth character of the ith element of field denotes the kind of vegetable that grows in the square at row i, column j  it's an uppercase letter ('A'  'Z'), where each letter denotes a different kind of vegetable. Return the area of the biggest plot we can buy.  
Definition  
 
Constraints  
  field will contain between 2 and 20 elements, inclusive.  
  Each element of field will contain between 2 and 20 characters, inclusive.  
  All elements of field will contain the same number of characters.  
  Each character in each element of field will be an uppercase letter ('A'  'Z').  
Examples  
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