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.


#include <stdio.h>
 
int semiprime(int n)
{
	int p, f = 0;
	for (p = 2; f < 2 && p*p <= n; p++)
		while (0 == n % p)
			n /= p, f++;
 
	return f + (n > 1) == 2;
}
 
int main(void)
{
	int i;
	for (i = 2; i < 100; i++)
		if (semiprime(i)) printf(" %d", i);
	putchar('\n');
 
	return 0;
}