Phase difference $(\varphi )$ and path difference $(\delta )$ are related by :

Relation between phase difference and path difference is
$\varphi =\frac{2\pi }{\lambda }\delta$
where $\lambda$ is the wavelength of the wave.
For constructive interference : 
Path difference  $\delta =n\lambda$  $⟹\varphi =2n\pi$
where n = 0, \ 1,\  2, \ ...
For destructive interference : 
Path difference  $\delta =(n+\frac{1}{2})\lambda$  $⟹\varphi =(2n+1)\pi$