1. def is_prime(n):                        
2.    if n <= 1:                          
3.        return False                    
4.    if n == 2 or n == 3:                
5.        return True                     
6.    if n % 2 == 0 or n % 3 == 0:         
7.        return False                    
8.    i = 5                               
9.    while i * i <= n:                   
10.       if n % i == 0 or n % (i + 2) == 0:
11.           return False                
12.       i += 6                           
13.   return True                         

Calculating the cyclomatic complexity purely by the formula: M = Decision Nodes + 1, one gets cyclomatic complexity as 6.
But, when I draw the control flow graph for the same and use the formula M = E - N + 2, I get the answer as 5. Why is there discrepancy?

P.S.: The nodes represent the line numbers. Nodes followed by an alphabet (4a, 4b, etc.) depict the two logical conditions about logical OR.