A semiprime is a natural number that is the product of two (not necessarily distinct) prime numbers.
Semiprimes (or biprimes): products of two primes http://oeis.org/A001358
Numbers which are not the sum of two semiprimes http://oeis.org/A072966
An interesting question: Is this sequence finite?
That is to say, can every natural number n > 33 be the sum of two semiprimes?