Physics Question about Diffraction Gradings

[tnbstudent]

Question:
What is the highest order of interference maximum that can occur when monochromatic light with a wavelength of 560 nm illuminates a diffraction grating with 4000 lines/cm?

Attempt:
I thought this seemed really easy, but my answer was incorrect.
λ=560nm
Since there are 4000 lines per centimeter - I solved for the distance between two slits to be 2.5*10^-6 meters.
Using I=mλ/d and m=1 to to achieve the max I got 0.224 but that was incorrect

I think I've made a silly error but I'm not sure what it is - can someone point me in a direction

 

[truesearch]

The max number of orders is d/λ which must be rounded down if it is not a whole number

 

[tnbstudent]

Thank you!