A Lucky number is a number which has only the digits 4 and 7. There's a special function F(n) for a positive integer n that returns all good digits of number n from the left to the right. For example, the output for number 147 is number 47. There are two numbers :
An integer a
A Lucky number b.
Find the minimum number c (c > a) such that F(c) = b.

Example 1:
Input:
a = 1, b = 7
Output:
7
Explanation:
The minimum number x greater than 1 such that
F(x) = 7, is 7 and so it is the answer.
Example 2:
Input:
a = 5, b = 4
Output:
14
Explanation:
The minimum number x greater than 5 such that
F(x) = 4, is 14 and so it is the answer.

Your Task:
You don't need to read input or print anything. Your task is to complete the function findTheNum() which takes 2 Integera a, and b as input and returns the minimum number x greater than a such that F(x) = b.

Expected Time Complexity: O(b)
Expected Auxiliary Space: O(1)

Constraints:
1 <= a,b <= 10^{5}

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