Problem Statement  
John thinks 4 and 7 are lucky digits, and all other digits are not lucky. A lucky number is a number that contains only lucky digits in decimal notation. You are given ints a and b. Return the number of lucky numbers between a and b, inclusive.  
Definition  
 
Constraints  
  a will be between 1 and 1,000,000,000, inclusive.  
  b will be between a and 1,000,000,000, inclusive.  
Examples  
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