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.
|-||a will be between 1 and 1,000,000,000, inclusive.|
|-||b will be between a and 1,000,000,000, inclusive.|