Interference fringes are observed on a screen by illuminating two thin slits 1 mm apart with a light source (λ=632.8 nm). The distance between the screen and the slits is 100 cm. Ifa bright fringe is observed on a screen at a distance of 1.27 mm from the central bright fringe, then the path difference between the waves, which are reaching this point from the slits is close to :