public static void main ( String [ ] args ) throws IOException {
  String input = ( new BufferedReader ( new InputStreamReader ( System . in ) ) ) . readLine ( ) ;
  int [ ] highs = Arrays . stream ( input . split ( " " ) ) . mapToInt ( Integer :: parseInt ) . toArray ( ) ;
  int diff = highs [ 1 ] - highs [ 0 ] ;
  int higher = IntStream . rangeClosed ( 1 , diff ) . sum ( ) ;
  System . out . println ( higher - highs [ 1 ] ) ;
}

public static void main ( String [ ] args ) throws IOException { String input = ( new BufferedReader ( new InputStreamReader ( System . in ) ) ) . readLine ( ) ; int [ ] highs = Arrays . stream ( input . split ( " ▁ " ) ) . mapToInt ( Integer :: parseInt ) . toArray ( ) ; int diff = highs [ 1 ] - highs [ 0 ] ; int higher = IntStream . rangeClosed ( 1 , diff ) . sum ( ) ; System . out . println ( higher - highs [ 1 ] ) ; }
(method_declaration (modifiers) type: (void_type) name: (identifier) parameters: (formal_parameters (formal_parameter type: (array_type element: (type_identifier) dimensions: (dimensions)) name: (identifier))) (throws (type_identifier)) body: (block (local_variable_declaration type: (type_identifier) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (parenthesized_expression (object_creation_expression type: (type_identifier) arguments: (argument_list (object_creation_expression type: (type_identifier) arguments: (argument_list (field_access object: (identifier) field: (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: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (string_literal))))) name: (identifier) arguments: (argument_list (method_reference (identifier) (identifier)))) name: (identifier) arguments: (argument_list)))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (binary_expression left: (array_access array: (identifier) index: (decimal_integer_literal)) right: (array_access array: (identifier) index: (decimal_integer_literal))))) (local_variable_declaration type: (integral_type) declarator: (variable_declarator name: (identifier) value: (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (decimal_integer_literal) (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: (identifier) right: (array_access array: (identifier) index: (decimal_integer_literal))))))))