public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  int m = 0 ;
  int [ ] [ ] ans ;
  if ( n % 2 == 0 ) {
    m = ( n - 2 ) * n / 2 ;
    ans = new int [ m ] [ 2 ] ;
    int c = 0 ;
    for ( int i = 1 ;
    i < n ;
    i ++ ) {
      for ( int j = i + 1 ;
      j <= n ;
      j ++ ) {
        if ( i + j == n + 1 ) continue ;
        ans [ c ] [ 0 ] = i ;
        ans [ c ] [ 1 ] = j ;
        c ++ ;
      }
    }
  }
  else {
    int n2 = n - 1 ;
    m = ( n2 - 2 ) * n2 / 2 + n2 ;
    ans = new int [ m ] [ 2 ] ;
    int c = 0 ;
    for ( int i = 1 ;
    i < n ;
    i ++ ) {
      for ( int j = i + 1 ;
      j <= n ;
      j ++ ) {
        if ( i + j == n ) continue ;
        ans [ c ] [ 0 ] = i ;
        ans [ c ] [ 1 ] = j ;
        c ++ ;
      }
    }
  }
  System . out . println ( m ) ;
  for ( int i = 0 ;
  i < m ;
  i ++ ) {
    System . out . println ( ans [ i ] [ 0 ] + " " + ans [ i ] [ 1 ] ) ;
  }
  sc . close ( ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; int m = 0 ; int [ ] [ ] ans ; if ( n % 2 == 0 ) { m = ( n - 2 ) * n / 2 ; ans = new int [ m ] [ 2 ] ; int c = 0 ; for ( int i = 1 ; i < n ; i ++ ) { for ( int j = i + 1 ; j <= n ; j ++ ) { if ( i + j == n + 1 ) continue ; ans [ c ] [ 0 ] = i ; ans [ c ] [ 1 ] = j ; c ++ ; } } } else { int n2 = n - 1 ; m = ( n2 - 2 ) * n2 / 2 + n2 ; ans = new int [ m ] [ 2 ] ; int c = 0 ; for ( int i = 1 ; i < n ; i ++ ) { for ( int j = i + 1 ; j <= n ; j ++ ) { if ( i + j == n ) continue ; ans [ c ] [ 0 ] = i ; ans [ c ] [ 1 ] = j ; c ++ ; } } } System . out . println ( m ) ; for ( int i = 0 ; i < m ; i ++ ) { System . out . println ( ans [ i ] [ 0 ] + " ▁ " + ans [ i ] [ 1 ] ) ; } sc . close ( ) ; }
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(identifier))) (expression_statement (assignment_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: (decimal_integer_literal)) right: (identifier))) (expression_statement (update_expression (identifier))))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (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: (identifier)) update: (update_expression (identifier)) body: (block (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (binary_expression left: (binary_expression left: (array_access array: (array_access array: (identifier) index: (identifier)) index: 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