How many photons hit 1 square metre aerial

Question

A question on an introductory text to quantum mechanics asks me to calculate the number of photons striking a $1$ metre square aerial each second at a distance of $1000km$ from a $200kW$ transmitter that broadcasts radio waves at a frequency of $200kHz$. I completely understand the solution apart from a small part where it says the aerial occupies $1$ square metre out of $4\pi (10^6)^2$. Where did the $4\pi$ come from???