The "R" and "L" in the combination should be interpreted as
moving right and left in the alphabet. After you guess the first
letter is A, the letter-number combinations give you the shifts to
successive letters.
1=A R17=18=R L15=3=C R5=8=H L7=1=A
R4=5=E R10=15=O L3=12=L R3=15=O
L8=7=G R2=9=I L6=3=C L2=1=A R11=12=L
The letters spell the answer ARCHAEOLOGICAL.