public static void main ( String [ ] args ) throws Exception {
  Scanner sc = new Scanner ( System . in ) ;
  String S = sc . next ( ) ;
  int [ ] ch = new int [ 26 ] ;
  for ( int i = 0 ;
  i < 26 ;
  i ++ ) ch [ i ] = 0 ;
  for ( int i = 0 ;
  i < S . length ( ) ;
  i ++ ) {
    int num = S . charAt ( i ) - 'a' ;
    ch [ num ] ++ ;
  }
  boolean state = true ;
  for ( int i = 0 ;
  i < 26 ;
  i ++ ) {
    if ( ch [ i ] > 1 ) {
      state = false ;
      break ;
    }
  }
  System . out . println ( state ? "yes" : "no" ) ;
}

public static void main ( String [ ] args ) throws Exception { Scanner sc = new Scanner ( System . in ) ; String S = sc . next ( ) ; int [ ] ch = new int [ 26 ] ; for ( int i = 0 ; i < 26 ; i ++ ) ch [ i ] = 0 ; for ( int i = 0 ; i < S . length ( ) ; i ++ ) { int num = S . charAt ( i ) - ' a ' ; ch [ num ] ++ ; } boolean state = true ; for ( int i = 0 ; i < 26 ; i ++ ) { if ( ch [ i ] > 1 ) { state = false ; break ; } } System . out . println ( state ? " yes " : " no " ) ; }
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