HackerRank ruby-enumerable-collect

# https://www.hackerrank.com/challenges/ruby-enumerable-collect
def rot(string, num)
  if num > 25
    num = num - (25 *(num/25))
  end
  new_UpFirst = (65 + num).chr
  new_UpLast = (65 + num - 1).chr
  new_DwnFirst = (97 + num).chr
  new_DwnLast = (97 + num - 1).chr
  case num
  when 0
    puts string
  when 1
    puts string.tr "A-Za-z", "#{new_UpFirst}-ZA#{new_DwnFirst}-za"
  when 25
    puts string.tr "A-Za-z", "ZA-#{new_UpLast}za-#{new_DwnLast}"
  else
    puts string.tr "A-Za-z", "#{new_UpFirst}-ZA-#{new_UpLast}#{new_DwnFirst}-za-#{new_DwnLast}"
  end
end