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.


var semiprime = Fn.new { |n|
    if (n < 3) return false
    var nf = 0
    for (i in 2..n) {
        while (n%i == 0) {
            if (nf == 2) return false
            nf = nf + 1
            n = (n/i).floor
        }
    }
    return nf == 2
}
 
for (v in 1675..1680) {
    System.print("%(v) -> %(semiprime.call(v) ? "is" : "is not") semi-prime")
}