function Main( input ){
	input = input.split( '\n' )
  var str = input[0]

  var book = {}
  var flg = true
  for( var i=0; i<str.length; i++ ){
    if( book[str[i]] === undefined ) book[str[i]] = true
    else {
      flg = false
      break
    }
  }

  console.log( flg ? 'yes' : 'no' )
}

Main( require( 'fs' ).readFileSync( '/dev/stdin', 'utf8' ) )
