ruby, 68 bytes

Result

WA

Duration

590ms

Code [DL]

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

stdin

BKMNDKCJSFUZRBXNRUUHGHKSJA

stdout

OPQRSTUVWXYZABCDEFGHIJKLMNO

stderr

strace