public static void main ( String args [ ] ) {
  Scanner sc = new Scanner ( System . in ) ;
  int num = sc . nextInt ( ) ;
  System . out . println ( num * ( num - 1 ) / 2 - num / 2 ) ;
  for ( int i = 1 ;
  i <= num ;
  i ++ ) {
    for ( int j = 1 ;
    j < i ;
    j ++ ) {
      if ( i + j == num / 2 * 2 + 1 ) {
        continue ;
      }
      System . out . println ( i + " " + j ) ;
    }
  }
}

public static void main ( String args [ ] ) { Scanner sc = new Scanner ( System . in ) ; int num = sc . nextInt ( ) ; System . out . println ( num * ( num - 1 ) / 2 - num / 2 ) ; for ( int i = 1 ; i <= num ; i ++ ) { for ( int j = 1 ; j < i ; j ++ ) { if ( i + j == num / 2 * 2 + 1 ) { continue ; } System . out . println ( i + " ▁ " + j ) ; } } }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (type_identifier) name: (identifier) dimensions: (dimensions))) 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: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (binary_expression left: (binary_expression left: (identifier) right: (parenthesized_expression (binary_expression left: (identifier) right: (decimal_integer_literal)))) right: (decimal_integer_literal)) right: (binary_expression left: (identifier) right: (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: (identifier)) update: (update_expression (identifier)) body: (block (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 (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (identifier)) right: (binary_expression left: (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (decimal_integer_literal)) right: (decimal_integer_literal)))) consequence: (block (continue_statement))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (binary_expression left: (identifier) right: (string_literal)) right: (identifier)))))))))))