def calc(input)
  original = ("a".."z").to_a
  a = original.difference(input.chars).first
  b = input.chars.select { |it| input.chars.count(it) > 1 }.first
  "#{a}to#{b}"
end

# input = "abcdefghijklmnopqrituvwxyz"
input = gets
puts calc(input)