public static void main ( String [ ] args ) {
  Scanner in = new Scanner ( System . in ) ;
  int n = in . nextInt ( ) ;
  in . nextLine ( ) ;
  String s = in . nextLine ( ) ;
  char [ ] a = s . toCharArray ( ) ;
  int ans = 0 ;
  for ( int i = 0 ;
  i < s . length ( ) ;
  i ++ ) {
    if ( a [ i ] >= 'a' && a [ i ] <= 'z' ) {
      hash [ a [ i ] - 'a' ] ++ ;
    }
    else {
      if ( hash [ a [ i ] - 'A' ] > 0 ) {
        hash [ a [ i ] - 'A' ] -- ;
      }
      else {
        ans ++ ;
      }
    }
  }
  System . out . println ( ans ) ;
}

public static void main ( String [ ] args ) { Scanner in = new Scanner ( System . in ) ; int n = in . nextInt ( ) ; in . nextLine ( ) ; String s = in . nextLine ( ) ; char [ ] a = s . toCharArray ( ) ; int ans = 0 ; for ( int i = 0 ; i < s . length ( ) ; i ++ ) { if ( a [ i ] >= ' a ' && a [ i ] <= ' z ' ) { hash [ a [ i ] - ' a ' ] ++ ; } else { if ( hash [ a [ i ] - ' A ' ] > 0 ) { hash [ a [ i ] - ' A ' ] -- ; } else { ans ++ ; } } } System . out . println ( ans ) ; }
(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)))) (expression_statement (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: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (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: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal)) right: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal)))) consequence: (block (expression_statement (update_expression (array_access array: (identifier) index: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal)))))) alternative: (block (if_statement condition: (parenthesized_expression (binary_expression left: (array_access array: (identifier) index: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal))) right: (decimal_integer_literal))) consequence: (block (expression_statement (update_expression (array_access array: (identifier) index: (binary_expression left: (array_access array: (identifier) index: (identifier)) right: (character_literal)))))) alternative: (block (expression_statement (update_expression (identifier))))))))) (expression_statement (method_invocation object: (field_access object: (identifier) field: (identifier)) name: (identifier) arguments: (argument_list (identifier))))))