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

def calc2(input)
  input.chars.zip(("a".."z")).select { |a, b| a != b }.first.then { |a, b| "#{b}to#{a}" }
end

# input = "abcdefghijklmnnpqrstuvwxyz"
input = gets
# puts calc(input)
puts calc2(input)