For constructive interference, the path difference between two interfering waves should be an integral multiple of the wavelength of wave i.e., path difference = nλ, n = 0, 1, 2, 3, ... and λ is the wavelength of the wave.
For destructive interference, the path difference between interfering waves should be half of odd number integral multiple of wavelength i.e., path difference = \(\left(2n+1\right)\frac\lambda2\), n = 0, 1, 2, 3, ...