Roman Numeral Converter

What are Roman numbers?
The Roman numerals are today a rarely used numbering system. Uppercase letters are made up to represent numbers. The joining of these letters is added or subtracted to make way for a different number.
All letters used are as follows:
I = 1
V = 5
X = 10
L= 50
C = 100
D= 500
M = 1000

How are Roman numerals written?
To write the Roman numerals you need to master addition and subtraction. To start you need to know that there can be no more than 3 equal letters in a row. If two letters are equal or the first is greater than the second, add, otherwise (where the first letter is smaller than the second), are subtracted.
For example:
I = 1
II = 1 + 1 = 2
III = 1 + 1 + 1 = 3
VII = 5 + 1 +1 = 7
IV = 5 – 1 = 4
IX = 10 - 1 = 9
XIX = 10 + 10 - 1 = 19