2 populations are independently surveyed using s.r.s of size n, and 2 proportions, p1 and p2, are estimated. It is expected that both population proportions are close to 0.5. What should the sample size be so that the standard error of the difference, will be less than 0.02?

I can find for the individual component, but I don't know how to apply for .

Answer is 1250.