Increase drop chances based on the number

Good day, there is a task. Take the number N (from 1 to 1000)

From a random function, we need a number R from 1 to 1,000,000

Moreover, if R = 950,000-990000. Then we get a unique item And if 990000-1000000 then a legendary item falls to us

Problem. Come up with math so that with an increase in N, the chance of dropping unique and legendary items increases.

I would like to hear your solutions, because I did not think of mine. Thank!