In my book this formula:

$$2\pi r=n\lambda$$

is mentioned only. However, its derivation or explanation has not been given. They only said, "So, the circumference of the circular path with radius $r$ will be the integral $(n)$ multiple of the wavelength ($\lambda$) of the moving electron."

However, they didn't give any explanation for it.

Could you please give an explanation as to why $2\pi r=n\lambda$ is true?