L is lambda..
And sorry i read the question properly now..we need to find no. Of minima
So -7l/2 ,-5l/2,-3l/2,-l/2,l/2,3l/2,5l/2,7l/2 will be the required path difference.
Note that the max path difference can be 4l and min path difference can be -4l( at +infty and -infty respectively)