Consider a series of numbers composed of only digits 4 and 7. First few numbers in the series are 4, 7, 44, 47, 74, 77, 444, .. etc. Given a string N constructed by 4, 7 digit only, find position of this number in this series.

Example 1:
Input: N = "7"
Output: 2
Explanation: In the series 4 , 7 , 44 , 47...
7 is at second position.

Example 2:
Input: N = "444"
Output: 7
Explanation: In the series 4 , 7 , 44 ,
47 , 74 , 77 , 444 , 447... 444 is at 7th
position

Your Task:
You don't need to read input or print anything. Your task is to complete the function findpos() which accepts a string N as input parameter and returns the position of that string in the given series.

Expected Time Complexity: O(Length of given string)
Expected Auxiliary Space: O(1)

Constraints:
1 <= Length of String < 50

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