def sum_series(n):
  
  if n < 1:
    return 0
  else:
    return n + sum_series(n - 2)
def is_not_prime(n):
    import math
    
    result = False
    for i in range(2,int(math.sqrt(n)) + 1):
        if n % i == 0:
            result = True
    return result