public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  String s = in . next ( ) ;
  k = in . nextInt ( ) ;
  Set < String > set = new HashSet < > ( ) ;
  Queue < String > pq = new PriorityQueue < > ( ) ;
  int len = s . length ( ) ;
  for ( int i = 0 ;
  i < len ;
  i ++ ) {
    int lim = i + k + 1 < len + 1 ? i + k + 1 : len + 1 ;
    for ( int j = i + 1 ;
    j < lim ;
    j ++ ) {
      String sub = s . substring ( i , j ) ;
      if ( ! set . contains ( sub ) ) {
        pq . add ( sub ) ;
        set . add ( sub ) ;
      }
    }
  }
  for ( int i = 0 ;
  i < k - 1 ;
  i ++ ) {
    pq . poll ( ) ;
  }
  print ( pq . peek ( ) ) ;
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; String s = in . next ( ) ; k = in . nextInt ( ) ; Set < String > set = new HashSet < > ( ) ; Queue < String > pq = new PriorityQueue < > ( ) ; int len = s . length ( ) ; for ( int i = 0 ; i < len ; i ++ ) { int lim = i + k + 1 < len + 1 ? i + k + 1 : len + 1 ; for ( int j = i + 1 ; j < lim ; j ++ ) { String sub = s . substring ( i , j ) ; if ( ! set . contains ( sub ) ) { pq . add ( sub ) ; set . add ( sub ) ; } } } for ( int i = 0 ; i < k - 1 ; i ++ ) { pq . poll ( ) ; } print ( pq . peek ( ) ) ; }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (field_access object: (identifier) field: (identifier)))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (expression_statement (assignment_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (generic_type (type_identifier) (type_arguments (type_identifier))) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (generic_type (type_identifier) (type_arguments)) arguments: (argument_list)))) (local_variable_declaration type: (generic_type (type_identifier) (type_arguments (type_identifier))) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (generic_type (type_identifier) (type_arguments)) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (ternary_expression condition: (binary_expression left: (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (decimal_integer_literal)) right: (binary_expression left: (identifier) right: (decimal_integer_literal))) consequence: (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (decimal_integer_literal)) alternative: (binary_expression left: (identifier) right: (decimal_integer_literal))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (identifier) right: (decimal_integer_literal)))) condition: (binary_expression left: (identifier) right: (identifier)) update: (update_expression (identifier)) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier) (identifier))))) (if_statement condition: (parenthesized_expression (unary_expression operand: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))))) consequence: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))))))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (binary_expression left: (identifier) right: (decimal_integer_literal))) update: (update_expression (identifier)) body: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))))) (expression_statement (method_invocation name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))))