Total words = 720
first 120 will start with A
for the next place first 24 will start with E
so first two letters are AE
in 24 words 6-6 will start with H, M, R, S
so required word will have first 3 letters as AER
next splitting 2-2 with H, M, S
so 18th word will have first 4 letters as AERS
so 17th word is AERSHM
and 18th word is AERSMH