Sum of Different Primes

A positive integer may be expressed as a sum of different prime numbers (primes), in one way or another. Given two positive integers $n$ and $k$, you should count the number of ways to express $n$ as a sum of $k$ different primes. Here, two ways are considered to be the same if they sum up the same set of the primes. For example, $8$ can be expressed as 3 + 5 and 5 + 3 but they are not distinguished.

When $n$ and $k$ are $24$ and $3$ respectively, the answer is two because there are two sets \{2, 3, 19\} and \{2, 5, 17\} whose sums are equal to $24$. There are no other sets of three primes that sum up to $24$. For n = 24 and k = 2, the answer is three, because there are three sets $\{5,19\}$, $\{7,17\}$ and $\{11,13\}$. For n = 2 and k = 1, the answer is one, because there is only one set $\{2\}$ whose sum is $2$. For n = 1 and k = 1, the answer is zero. As $1$ is not a prime, you shouldn't count $\{1\}$. For n = 4 and k = 2, the answer is zero, because there are no sets of two different primes whose sums are $4$.

Your job is to write a program that reports the number of such ways for the given $n$ and $k$.