public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int N = sc . nextInt ( ) ;
  int M = sc . nextInt ( ) ;
  TreeMap < Long , Integer > map = new TreeMap < > ( ) ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    long cost = sc . nextLong ( ) ;
    int num = sc . nextInt ( ) ;
    map . put ( cost , map . containsKey ( cost ) ? num + map . get ( cost ) : num ) ;
  }
  sc . close ( ) ;
  long totalcost = 0 ;
  int bought = 0 ;
  for ( Map . Entry < Long , Integer > kv : map . entrySet ( ) ) {
    long cost = kv . getKey ( ) ;
    int maxnum = kv . getValue ( ) ;
    int buycount = Math . min ( maxnum , M - bought ) ;
    totalcost += buycount * cost ;
    bought += buycount ;
    if ( bought > M ) {
      break ;
    }
  }
  System . out . println ( totalcost ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int N = sc . nextInt ( ) ; int M = sc . nextInt ( ) ; TreeMap < Long , Integer > map = new TreeMap < > ( ) ; for ( int i = 0 ; i < N ; i ++ ) { long cost = sc . nextLong ( ) ; int num = sc . nextInt ( ) ; map . put ( cost , map . containsKey ( cost ) ? num + map . get ( cost ) : num ) ; } sc . close ( ) ; long totalcost = 0 ; int bought = 0 ; for ( Map . Entry < Long , Integer > kv : map . entrySet ( ) ) { long cost = kv . getKey ( ) ; int maxnum = kv . getValue ( ) ; int buycount = Math . min ( maxnum , M - bought ) ; totalcost += buycount * cost ; bought += buycount ; if ( bought > M ) { break ; } } System . out . println ( totalcost ) ; }
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