Sum of Primes

$f$ is a function such that $f(x)$ equals the number of ways in which $x$ can be expressed as sum of two prime numbers. For example $f(10) = 2$, as 10 can be expressed as sum of two primes in two ways: $ 3 + 7 = 5 + 5 = 10$. More formally, $f(x)$ is the number of pairs $(a, b)$ such that $0\le a \le b < x$ and $a$ and $b$ are prime and $a + b = x$. Given an integer $N$, find the number of pairs