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Reverse to old CheckiO mission Roman Numerals
Input: roman numeral as string
Output: Int.
Example:
checkio('VI') == 6
checkio('LXXVI') == 76
checkio('CDXCIX') == 499
checkio('MMMDCCCLXXXVIII') == 3888
Precondition: Non empty string that can contain following symbols: I,V,X,C,D,M,L