I = 1
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1000
Add values when a smaller numeral comes after a larger one
Subtract values when a smaller numeral comes before a larger one
IV = 4
IX = 9
XL = 40
XC = 90
CD = 400
CM = 900
Read from left to right
Combine the values of each symbol using the add or subtract rule