public static void main ( String [ ] args ) {
  String s = sc . next ( ) ;
  int n = nextInt ( ) ;
  ArrayList < String > list = new ArrayList < > ( ) ;
  for ( int i = 0 ;
  i < s . length ( ) ;
  i ++ ) {
    for ( int j = 0 ;
    j < s . length ( ) ;
    j ++ ) {
      StringBuilder temp = new StringBuilder ( ) ;
      temp . append ( s . charAt ( i ) ) ;
      temp . append ( s . charAt ( j ) ) ;
      list . add ( temp . toString ( ) ) ;
    }
  }
  Collections . sort ( list ) ;
  out . println ( list . get ( n - 1 ) ) ;
}

public static void main ( String [ ] args ) { String s = sc . next ( ) ; int n = nextInt ( ) ; ArrayList < String > list = new ArrayList < > ( ) ; for ( int i = 0 ; i < s . length ( ) ; i ++ ) { for ( int j = 0 ; j < s . length ( ) ; j ++ ) { StringBuilder temp = new StringBuilder ( ) ; temp . append ( s . charAt ( i ) ) ; temp . append ( s . charAt ( j ) ) ; list . add ( temp . toString ( ) ) ; } } Collections . sort ( list ) ; out . println ( list . get ( n - 1 ) ) ; }
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