ruby, 75 bytes

Result

WA

Duration

571ms

Code [DL]

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

stdin

STULDSAZDZKKWIDGRPJSWTEWSZ

stdout

OPQRSTUVWXYZABCDEFGHIJKLMN

stderr

strace