# Count Binary Strings

easy

1. You are given a number n. 2. You are required to print the number of binary strings of length n with no consecutive 0's.

## Constraints

0 < n <= 45

## Format

### Input

A number n

### Output

A number representing the number of binary strings of length n with no consecutive 0's.

## Example

Sample Input

6

### Sample Output

21

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