5.2 Dexter and D-numbers
(RankList for this Question)
Score: 60pts
Time Limit: 1.00 sec
Dexter in his laboratory was working on new numbers which he would call D-Numbers. D-number is a number which consists of only 1s and 2s. For eg - 1, 2, 12, 21 are D-numbers however 101, 72, 128 are not.
One fine day, DeDe walked into Dexter's lab and gave him a D-number \(X\) and asked him how many D-numbers exist which are smaller than the number \(X\). Can you help Dexter answer this question?

Constraints
\(1 \leq X \leq 10^9\)

Input Format
\(X\)

Output Format
\(N\)
\(N\) is the count of D-numbers smaller than X.

Example 1
Input:
1

Output:
0

Explanation:
There is no D-number which is smaller than 1.

Example 2
Input:
22

Output:
5

Explanation:
The 5 D-numbers smaller than 22 are - 1, 2, 11, 12, 21.

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