Look at old latin inscriptions in Europe and you might see IIII instead of IV. Turns out the rules of Roman numerals we know today are a recent convention :)
When you want to add two roman numbers, you just i. put them together, ii. sort the digits in descending order, and iii. shorten all "iiiii" to "v", all "vv" to "x", and so on.
i. vii + xiii -> viixiii
ii. viixiii -> xviiiii
iii. xviiiii -> xvv -> xx