public static void main ( String [ ] args ) throws IOException {
  File f = new File ( "input.txt" ) ;
  FileReader fileReader = new FileReader ( f ) ;
  BufferedReader reader = new BufferedReader ( fileReader ) ;
  int n = Integer . parseInt ( reader . readLine ( ) ) ;
  StringBuilder sb = new StringBuilder ( ) ;
  StringTokenizer st = new StringTokenizer ( reader . readLine ( ) ) ;
  int [ ] [ ] number = new int [ 5001 ] [ 2 ] ;
  for ( int i = 1 ;
  i <= 2 * n ;
  i ++ ) {
    int temp = Integer . parseInt ( st . nextToken ( ) ) ;
    if ( number [ temp ] [ 0 ] > 0 ) {
      sb . append ( number [ temp ] [ 1 ] + " " + i + "\n" ) ;
      number [ temp ] [ 0 ] = 0 ;
    }
    else {
      number [ temp ] [ 0 ] ++ ;
      number [ temp ] [ 1 ] = i ;
    }
  }
  FileWriter writer = new FileWriter ( "output.txt" ) ;
  for ( int i = 1 ;
  i < 5001 ;
  i ++ ) {
    if ( number [ i ] [ 0 ] > 0 ) {
      sb = new StringBuilder ( ) ;
      sb . append ( "-1" ) ;
      break ;
    }
  }
  writer . write ( String . valueOf ( sb ) ) ;
  writer . close ( ) ;
}

public static void main ( String [ ] args ) throws IOException { File f = new File ( " input . txt " ) ; FileReader fileReader = new FileReader ( f ) ; BufferedReader reader = new BufferedReader ( fileReader ) ; int n = Integer . parseInt ( reader . readLine ( ) ) ; StringBuilder sb = new StringBuilder ( ) ; StringTokenizer st = new StringTokenizer ( reader . readLine ( ) ) ; int [ ] [ ] number = new int [ 5001 ] [ 2 ] ; for ( int i = 1 ; i <= 2 * n ; i ++ ) { int temp = Integer . parseInt ( st . nextToken ( ) ) ; if ( number [ temp ] [ 0 ] > 0 ) { sb . append ( number [ temp ] [ 1 ] + " ▁ " + i + " \n " ) ; number [ temp ] [ 0 ] = 0 ; } else { number [ temp ] [ 0 ] ++ ; number [ temp ] [ 1 ] = i ; } } FileWriter writer = new FileWriter ( " output . txt " ) ; for ( int i = 1 ; i < 5001 ; i ++ ) { if ( number [ i ] [ 0 ] > 0 ) { sb = new StringBuilder ( ) ; sb . append ( " - 1" ) ; break ; } } writer . write ( String . valueOf ( sb ) ) ; writer . close ( ) ; }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) (throws (type_identifier)) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (string_literal))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (identifier))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (identifier))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list)))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))) (local_variable_declaration type: (array_type element: (integral_type) dimensions: (dimensions)) declarator: (variable_declarator name: (identifier) value: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (decimal_integer_literal)) dimensions: (dimensions_expr (decimal_integer_literal))))) (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: (decimal_integer_literal) right: (identifier))) update: (update_expression (identifier)) body: (block (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))) (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (block (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (binary_expression left: (binary_expression left: (binary_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (decimal_integer_literal)) right: (string_literal)) right: (identifier)) right: (string_literal))))) (expression_statement (assignment_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (decimal_integer_literal)) right: (decimal_integer_literal)))) alternative: (block (expression_statement (update_expression (array_access array: (array_access array: (identifier) index: (identifier)) index: (decimal_integer_literal)))) (expression_statement (assignment_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (decimal_integer_literal)) right: (identifier))))))) (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (type_identifier) arguments: (argument_list (string_literal))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (decimal_integer_literal)) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (block (expression_statement (assignment_expression left: (identifier) right: (object_creation_expression type: (type_identifier) arguments: (argument_list)))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (string_literal)))) (break_statement))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))))) (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))