Reverse Roman Numerals

Reverse Roman Numerals

In this CheckiO mission Roman Numerals you have to convert a decimal number into its representation as a Roman number.
Here you have to do the same but the other way around.

You are given a Roman number as a string and your job is to convert this number into ...

NumeralValue
I1 (unus)
V5 (quinque)
X10 (decem)
L50 (quinquaginta)
C100 (centum)
D500 (quingenti)
M1,000 (mille)

Input: A Roman number as a string.

Output: The decimal representation of the Roman number as an int.

Example:

reverseRoman('VI') == 6
reverseRoman('LXXVI') == 76
reverseRoman('CDXCIX') == 499
reverseRoman('MMMDCCCLXXXVIII') == 3888

Precondition:
len(roman_string) > 0
all(char in "MDCLXVI" for char in roman_string) == True
0 < reverse_roman(roman_string) < 4000

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