final romanNumbers = {
    "I": 1,
    "V": 5,
    "X": 10,
    "L": 50,
    "C": 100,
    "D": 500,
    "M": 1000
  };  

void main() 
{
    print('III: ${romanToInt('III')}');
    print('IV: ${romanToInt('IV')}');
    print('IX: ${romanToInt('IX')}');
    print('LVIII: ${romanToInt('LVIII')}');
    print('MCMXCIV: ${romanToInt('MCMXCIV')}');
}

int romanToInt(String roman)
{
	return roman.split('')
				.reversed
				.map((n) => romanNumbers[n])
				.reduce((curr, next) => curr > 2 * next ? curr - next : curr + next);
}