212. Roman Numeral Decoder

Write a function solve that converts a Roman numeral string to its integer value. Roman numerals use: I=1, V=5, X=10, L=50, C=100, D=500, M=1000. Subtractive cases: IV=4, IX=9, XL=40, XC=90, CD=400, CM=900.

Example:
Input: "MCMXC"
Output: 1990

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