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This tag indicates that the challenge involves Roman numerals

This tag indicates that the challenge involves Roman numerals. Roman numerals are a compact way of representing numbers in unary.

The basics of Roman numerals:

In Roman numerals, I stands for 1, V for 5, X:10, L:50, C:100, D:500, M:1000. A useful mnemonic for remember the values greater than 10 is Let Caesar Destroy Mountains (LCDM).

From Wikipedia:

Numbers are formed by combining symbols together and adding the values. So II is two ones, i.e. 2, and XIII is a ten and three ones, i.e. 13. There is no zero in this system, so 207, for example, is CCVII, using the symbols for two hundreds, a five and two ones. 1066 is MLXVI, one thousand, fifty and ten, a five and a one.

In the standard form of roman numerals:

Symbols are placed from left to right in order of value, starting with the largest. However, in a few specific cases, to avoid four characters being repeated in succession (such as IIII or XXXX) these can be reduced using subtractive notation as follows:

the numeral I can be placed before V and X to make 4 units (IV) and 9 units (IX respectively)

X can be placed before L and C to make 40 (XL) and 90 (XC respectively)

C can be placed before D and M to make 400 (CD) and 900 (CM) according to the same pattern

An example using the above rules would be 1904: this is composed of 1 (one thousand), 9 (nine hundreds), 0 (zero tens), and 4 (four units). To write the Roman numeral, each of the non-zero digits should be treated separately. Thus 1,000 = M, 900 = CM, and 4 = IV. Therefore, 1904 is MCMIV.