import Data.List

main::IO()
main=do
  str<-getLine
  putStr (solver str)


solver::String->String
solver str = let sym = map (\ls -> head ls) (group (sort str)) in
             let ls  = filter (\x -> not (elem x sym)) ['a'..'z'] in
             if ls ==[] then "None\n" else (head ls):"\n"