Considering the positions of letters, we get A = 1 : Z = 26
Again in the next case, B = 2 : Y = 25, which again sums to 27.
Similarly, K = 11, so we need 16, which is P.
L = 12, so we need 15, which is O.
M = 13, so we need 14, which is N.
N = 14, so we need 13, which is M.
So, the code should be PONM.