You simply change each of the numbers to its Roman equivalent, keeping the dashes. This would be shown as VI-XXVI-MMIX.
12 = XII in Roman numerals
The Roman numerals are XLV.
18 in roman numerals looks like XVIII
300 would be represented by 'CCC' in roman numerals.
MDCCCXCV
MMDCCCLXXXVI
MCMLXXXV
MCMLXV
XL
MCMLII
mmxiii
D1X