crack.rb

#!/usr/bin/env ruby

if ?A.is_a? String
  ab = (?A..?Z).to_a
else
  ab = (?A..?Z).map {|c| c.chr}
end
n = $<.readline.to_i
str = $<.read
puts str.split('').map {|oc| c = oc.upcase; if ab.include? c then r = ab[(ab.index(c) - n) % ab.length]; c == oc ? r : r.downcase else c end }.join