Study the following series.
2 S E 9 1 K L C O $ H 5 Z 4 8 U X W 6 @
Find what should come in place of the question mark(?)
SE , 1L , ? , H8
There is a gap of 2 between the first characters of each of the given pair. The second character of each pair is increasingly moving one position to the right of the first character.