public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  ArrayList < Data > list = new ArrayList < Data > ( ) ;
  String [ ] line_list = sc . nextLine ( ) . split ( " " ) ;
  int N = Integer . parseInt ( line_list [ 0 ] ) ;
  int M = Integer . parseInt ( line_list [ 1 ] ) ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    line_list = sc . nextLine ( ) . split ( " " ) ;
    list . add ( new Data ( Long . parseLong ( line_list [ 0 ] ) , Integer . parseInt ( line_list [ 1 ] ) ) ) ;
  }
  list . sort ( ( a , b ) -> a . compareTo ( b ) ) ;
  long kingaku = 0 ;
  for ( int i = 0 ;
  i < list . size ( ) ;
  i ++ ) {
    Data data = list . get ( i ) ;
    if ( M < data . getSuryou ( ) ) {
      kingaku = kingaku + M * data . getTanka ( ) ;
      break ;
    }
    else {
      kingaku = kingaku + data . getSuryou ( ) * data . getTanka ( ) ;
      M = M - data . getSuryou ( ) ;
    }
  }
  System . out . println ( kingaku ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; ArrayList < Data > list = new ArrayList < Data > ( ) ; String [ ] line_list = sc . nextLine ( ) . split ( " ▁ " ) ; int N = Integer . parseInt ( line_list [ 0 ] ) ; int M = Integer . parseInt ( line_list [ 1 ] ) ; for ( int i = 0 ; i < N ; i ++ ) { line_list = sc . nextLine ( ) . split ( " ▁ " ) ; list . add ( new Data ( Long . parseLong ( line_list [ 0 ] ) , Integer . parseInt ( line_list [ 1 ] ) ) ) ; } list . sort ( ( a , b ) -> a . compareTo ( b ) ) ; long kingaku = 0 ; for ( int i = 0 ; i < list . size ( ) ; i ++ ) { Data data = list . get ( i ) ; if ( M < data . getSuryou ( ) ) { kingaku = kingaku + M * data . getTanka ( ) ; break ; } else { kingaku = kingaku + data . getSuryou ( ) * data . getTanka ( ) ; M = M - data . getSuryou ( ) ; } } System . out . println ( kingaku ) ; }
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