Semiprime numbers are natural numbers that are products of exactly two (possibly equal) prime numbers.


Semiprimes   are also known as:

   semi-primes
   biprimes
   bi-primes
    2-almost    primes
   or simply:    P2  


Example
 
   1679  =  23 × 73  

(This particular number was chosen as the length of the Arecibo message).



Task

Write a function determining whether a given number is semiprime.



See also

 The Wikipedia article:  semiprime.
 The Wikipedia article:  almost prime.
 The OEIS sequence:  A001358: semiprimes  which has a shorter definition: the product of two primes.


 
;;;	Practically identical to the EchoLisp solution
(define (semiprime? n)
	(= (length (factor n)) 2))
;
;;;	Example (sadly factor doesn't accept bigints)
(println (filter semiprime? (sequence 2 100)))
(setq x 9223372036854775807)
(while (not (semiprime? x)) (-- x))
(println "Biggest semiprime reachable: " x " = " ((factor x) 0) " x " ((factor x) 1))
 