ruby, 64 bytes

Result

WA

Duration

593ms

Code [DL]

p gets.chars.map.with_index{|c,i|((c.to_i+i-12)%26+65).chr}.join

stdin

FYTMEKAIDQRRCMMSRKGNEDKOMT

stdout

"OPQRSTUVWXYZABCDEFGHIJKLMNO"

stderr

strace