Strings — Roman Numeral to Integer

solve(s) — convert a valid Roman numeral (1..3999) to an integer. The symbols are
I=1 V=5 X=10 L=50 C=100 D=500 M=1000. Symbols are normally written largest to smallest and
added, except in six subtractive cases (IV=4 IX=9 XL=40 XC=90 CD=400 CM=900) where a
smaller symbol before a larger one is subtracted.

Scan left to right: when a symbol is smaller than the one after it, subtract it,
otherwise add it.

Examples

solve("III")       → 3
solve("LVIII")     → 58
solve("MCMXCIV")   → 1994