public static void main ( String [ ] args ) {
  ArrayList < String > array = new ArrayList < String > ( ) ;
  ArrayList < String > arraySub ;
  Scanner scan = new Scanner ( System . in ) ;
  String substring = scan . nextLine ( ) ;
  int outNum = scan . nextInt ( ) ;
  for ( int i = 0 ;
  i < substring . length ( ) ;
  i ++ ) {
    String subText = substring . substring ( i ) ;
    for ( int j = 0 ;
    j < subText . length ( ) && j < outNum ;
    j ++ ) {
      array . add ( subText . substring ( 0 , j + 1 ) ) ;
    }
  }
  arraySub = new ArrayList < String > ( new HashSet < > ( array ) ) ;
  Collections . sort ( arraySub ) ;
  System . out . println ( arraySub . get ( outNum - 1 ) ) ;
}

public static void main ( String [ ] args ) { ArrayList < String > array = new ArrayList < String > ( ) ; ArrayList < String > arraySub ; Scanner scan = new Scanner ( System . in ) ; String substring = scan . nextLine ( ) ; int outNum = scan . nextInt ( ) ; for ( int i = 0 ; i < substring . length ( ) ; i ++ ) { String subText = substring . substring ( i ) ; for ( int j = 0 ; j < subText . length ( ) && j < outNum ; j ++ ) { array . add ( subText . substring ( 0 , j + 1 ) ) ; } } arraySub = new ArrayList < String > ( new HashSet < > ( array ) ) ; Collections . sort ( arraySub ) ; System . out . println ( arraySub . get ( outNum - 1 ) ) ; }
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