ruby, 74 bytes

Result

WA

Duration

553ms

Code [DL]

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

stdin

BDGRJMYDDFOPCVMQDCFNSLQGMG

stdout

OPQRSTUVWXYZABCDEFGHIJKLMN

stderr

strace