public static void main ( String [ ] args ) {
  Scanner scn = new Scanner ( System . in ) ;
  int n = scn . nextInt ( ) ;
  String str = scn . next ( ) ;
  int [ ] r = new int [ 2 ] ;
  int [ ] b = new int [ 2 ] ;
  for ( int i = 0 ;
  i < str . length ( ) ;
  i ++ ) {
    if ( str . charAt ( i ) == 'r' ) {
      r [ i % 2 ] ++ ;
    }
    else {
      b [ i % 2 ] ++ ;
    }
  }
  System . out . println ( Math . min ( Math . max ( r [ 0 ] , b [ 1 ] ) , Math . max ( r [ 1 ] , b [ 0 ] ) ) ) ;
}

public static void main ( String [ ] args ) { Scanner scn = new Scanner ( System . in ) ; int n = scn . nextInt ( ) ; String str = scn . next ( ) ; int [ ] r = new int [ 2 ] ; int [ ] b = new int [ 2 ] ; for ( int i = 0 ; i < str . length ( ) ; i ++ ) { if ( str . charAt ( i ) == ' r ' ) { r [ i % 2 ] ++ ; } else { b [ i % 2 ] ++ ; } } System . out . println ( Math . min ( Math . max ( r [ 0 ] , b [ 1 ] ) , Math . max ( r [ 1 ] , b [ 0 ] ) ) ) ; }
(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: (type_identifier) 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: (array_creation_expression type: (integral_type) dimensions: (dimensions_expr (decimal_integer_literal))))) (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))))) (for_statement init: (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (decimal_integer_literal))) condition: (binary_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))) update: (update_expression (identifier)) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))) right: (character_literal))) consequence: (block (expression_statement (update_expression (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal)))))) alternative: (block (expression_statement (update_expression (array_access array: (identifier) index: (binary_expression left: (identifier) right: (decimal_integer_literal))))))))) (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 (array_access array: (identifier) index: (decimal_integer_literal)) (array_access array: (identifier) index: (decimal_integer_literal)))) (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (array_access array: (identifier) index: (decimal_integer_literal)) (array_access array: (identifier) index: (decimal_integer_literal)))))))))))