Find the previous fibonacci number

Given a Fibonacci number N, the task is to find the previous Fibonacci number.
Examples: 
 

Input: N = 8 
Output: 5 
5 is the previous fibonacci number before 8.
Input: N = 5 
Output: 3 
 

Approach: The ratio of two adjacent numbers in the Fibonacci series rapidly approaches ((1 + sqrt(5)) / 2). So if N is divided by ((1 + sqrt(5)) / 2) and then rounded, the resultant number will be the previous Fibonacci number. 
Below is the implementation of the above approach: 
 

5

Time Complexity: O(1)

Auxiliary Space: O(1)