Convert between Roman numerals (I, V, X, L, C, D, M) and decimal numbers.
Roman numerals use letters to represent values. When a smaller value precedes a larger one, it's subtracted (IV = 4). When it follows, it's added (VI = 6). The subtractive principle only applies to certain combinations: I before V/X, X before L/C, C before D/M.
Using standard notation, the largest number is 3,999 (MMMCMXCIX). Extended notation uses bars over letters to multiply by 1,000, allowing much larger numbers.