518391
Oh it's not allll that bad! Other than knowing what each letter represents, there are a few rules to know about !
Like you can't have more than 3 of a roman numeral letter in a row. You would have to substitute when needed to do so. 4 becomes IV (one less than 5), 9 becomes IX (one less than 10), 40 becomes XL, 400 becomes CD (one hundred less than 500). 900 would be CM.
You however would not do this if unnecessary, like 49 is not IL, 99 isn't IC, etc.