When you explained the solution to their problem by drawing this diagram, Shubhlin asked you: 'Why have you coloured all the stations' locations with different shades?' You answered: The colour shades become darker -
as the numbers increase in order
The shades of the colour become darker as the numbers increase in order. This is because the numbers on the number line increase from left to right. Thus, 1 is the smallest positive integer and -1 is the largest negative integer.