public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int n = sc . nextInt ( ) ;
  int m = sc . nextInt ( ) ;
  BigDecimal sum = BigDecimal . ZERO ;
  List < Entry < Integer , Integer >> list = new ArrayList < > ( ) ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) list . add ( new SimpleEntry < Integer , Integer > ( sc . nextInt ( ) , sc . nextInt ( ) ) ) ;
  list = list . stream ( ) . sorted ( Comparator . comparingInt ( entry -> entry . getKey ( ) ) ) . collect ( Collectors . toList ( ) ) ;
  for ( Entry < Integer , Integer > entry : list ) {
    if ( m == 0 ) break ;
    BigDecimal a = BigDecimal . valueOf ( entry . getKey ( ) ) ;
    BigDecimal b = BigDecimal . valueOf ( entry . getValue ( ) ) ;
    if ( m > entry . getValue ( ) ) {
      m -= entry . getValue ( ) ;
      sum = sum . add ( a . multiply ( b ) ) ;
    }
    else if ( m <= entry . getValue ( ) ) {
      b = BigDecimal . valueOf ( m ) ;
      sum = sum . add ( a . multiply ( b ) ) ;
      m = 0 ;
    }
  }
  System . out . println ( sum . toString ( ) ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int n = sc . nextInt ( ) ; int m = sc . nextInt ( ) ; BigDecimal sum = BigDecimal . ZERO ; List < Entry < Integer , Integer > > list = new ArrayList < > ( ) ; for ( int i = 0 ; i < n ; i ++ ) list . add ( new SimpleEntry < Integer , Integer > ( sc . nextInt ( ) , sc . nextInt ( ) ) ) ; list = list . stream ( ) . sorted ( Comparator . comparingInt ( entry -> entry . getKey ( ) ) ) . collect ( Collectors . toList ( ) ) ; for ( Entry < Integer , Integer > entry : list ) { if ( m == 0 ) break ; BigDecimal a = BigDecimal . valueOf ( entry . getKey ( ) ) ; BigDecimal b = BigDecimal . valueOf ( entry . getValue ( ) ) ; if ( m > entry . getValue ( ) ) { m -= entry . getValue ( ) ; sum = sum . add ( a . multiply ( b ) ) ; } else if ( m <= entry . getValue ( ) ) { b = BigDecimal . valueOf ( m ) ; sum = sum . add ( a . multiply ( b ) ) ; m = 0 ; } } System . out . println ( sum . toString ( ) ) ; }
(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: (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: (field_access object: (identifier) field: (identifier)))) (local_variable_declaration type: (generic_type (type_identifier) (type_arguments (generic_type (type_identifier) (type_arguments (type_identifier) (type_identifier))))) declarator: (variable_declarator name: (identifier) value: (object_creation_expression type: (generic_type (type_identifier) (type_arguments)) arguments: (argument_list)))) (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: (expression_statement (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (object_creation_expression type: (generic_type (type_identifier) (type_arguments (type_identifier) (type_identifier))) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))))) (expression_statement (assignment_expression left: (identifier) right: (method_invocation object: (method_invocation object: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (lambda_expression parameters: (identifier) body: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list))))))) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))))) (enhanced_for_statement type: (generic_type (type_identifier) (type_arguments (type_identifier) (type_identifier))) name: (identifier) value: (identifier) body: (block (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: 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(expression_statement (assignment_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier)))))))) alternative: (if_statement condition: (parenthesized_expression (binary_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list)))) consequence: (block (expression_statement (assignment_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))))) (expression_statement (assignment_expression left: (identifier) right: (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (method_invocation object: (identifier) name: (identifier) arguments: (argument_list (identifier))))))) (expression_statement (assignment_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)))))))