Greater Dice

You have two fair dice. One of them is $m-$sided, while the other is $n-$sided. The dice have respective values $1-m$ and $1-n$ on the sides. Suppose that $n > m$. Find the probability that the $n-$sided die shows a strictly larger value than the $m-$sided die. Solve this with $m = 20$ and $n = 30$.