public static void main ( String [ ] args ) {
  Scanner input = new Scanner ( System . in ) ;
  int n = input . nextInt ( ) ;
  char [ ] arr = input . next ( ) . toCharArray ( ) ;
  int evenB = ( n + 1 ) / 2 , evenR = ( n + 1 ) / 2 ;
  int oddB = n / 2 , oddR = n / 2 ;
  for ( int i = 0 ;
  i < arr . length ;
  i ++ ) {
    if ( i % 2 == 0 ) {
      if ( arr [ i ] == 'r' ) evenR -- ;
      else evenB -- ;
    }
    else {
      if ( arr [ i ] == 'r' ) oddR -- ;
      else oddB -- ;
    }
  }
  System . out . println ( Math . min ( Math . max ( evenB , oddR ) , Math . max ( evenR , oddB ) ) ) ;
}

public static void main ( String [ ] args ) { Scanner input = new Scanner ( System . in ) ; int n = input . nextInt ( ) ; char [ ] arr = input . next ( ) . toCharArray ( ) ; int evenB = ( n + 1 ) / 2 , evenR = ( n + 1 ) / 2 ; int oddB = n / 2 , oddR = n / 2 ; for ( int i = 0 ; i < arr . length ; i ++ ) { if ( i % 2 == 0 ) { if ( arr [ i ] == ' r ' ) evenR -- ; else evenB -- ; } else { if ( arr [ i ] == ' r ' ) oddR -- ; else oddB -- ; } } System . out . println ( Math . min ( Math . max ( evenB , oddR ) , Math . max ( evenR , oddB ) ) ) ; }
(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: (integral_type) declarator: (variable_declarator name: (identifier) value: (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: (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (parenthesized_expression (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (decimal_integer_literal))) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (parenthesized_expression (binary_expression left: (identifier) right: (decimal_integer_literal))) right: (decimal_integer_literal)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (identifier) right: (decimal_integer_literal))) declarator: (variable_declarator name: (identifier) value: (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: (field_access object: (identifier) field: (identifier))) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (binary_expression left: (identifier) right: (decimal_integer_literal)) right: (decimal_integer_literal))) consequence: (block (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal))) consequence: (expression_statement (update_expression (identifier))) alternative: (expression_statement (update_expression (identifier))))) alternative: (block (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal))) consequence: (expression_statement (update_expression (identifier))) alternative: (expression_statement (update_expression (identifier)))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier) (identifier))) (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier) (identifier))))))))))