roma numbers

Book volumes use capital Roman numerals. (I, II, III, IV, V, VI, VII, VIII, IX, X)
In a book, the pages before chapter 1 use lower case Roman numbers (i, ii, iii, iv, v, etc)

There are 3 rules:
- no more than 3 of the same letter in a row (3 is III, 4 is IV)
- the letter before is subtracted 4 = IV (5 - 1)
- the letter after is added 6 = VI (5 + 1)

20 = XX, 30 = XXX, 40 = XL, 50 = L, 60 = LX, 100 = C, 300 = CCC, 500 = D, 1000 = M, 2000 = MM