ruby, 74 bytes

Result

WA

Duration

574ms

Code [DL]

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

stdin

OIIZARSWPGMJCYEORFYNWSVPAK

stdout

stderr

/volume/CODE:1:in `+': String can't be coerced into Integer (TypeError)
	from /volume/CODE:1:in `block in <main>'
	from /volume/CODE:1:in `map'
	from /volume/CODE:1:in `with_index'
	from /volume/CODE:1:in `<main>'

strace