Choosing order number with interference.

[going2breakdown]

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I have a problem here:
A way to make an airplane invisible to radar would be to coat it with an antireflective polymer. Radar waves have a wavelength of 3 cm and the index of refraction for the polymer is n=1.5. How thick should the coating be?
The answer is 0.5 cm. I understand the equation here but I'm not sure why we chose the order number 0. Why don't we use higher order numbers to calculate the destructive wavelength thickness.
i.e.
2nt= (m+ 0.5) lambda. why is m zero in this case? why can't we also use m=1? At m=1 it is still destructive interference. Is it because we want the thinnest possible layer?

 

[FeinMS]

The question is poorly written. It should've specified that it's looking for the MINIMUM thickness. There are several thickness values that results in destructive interference for such wavelength (found by manipulating order number,) To find the minimum thickness, you should use the smallest m value that results in non-negative, non-zero thickness.