public static void main ( String [ ] args ) {
  Scanner scanner = new Scanner ( System . in ) ;
  int m = scanner . nextInt ( ) ;
  int k = scanner . nextInt ( ) ;
  int [ ] length = new int [ m ] ;
  int [ ] oil = new int [ m ] ;
  for ( int i = 0 ;
  i < m ;
  i ++ ) {
    length [ i ] = scanner . nextInt ( ) ;
  }
  for ( int i = 0 ;
  i < m ;
  i ++ ) {
    oil [ i ] = scanner . nextInt ( ) ;
  }
  int maxOil = 0 ;
  int costTime = 0 ;
  int oilLeft = 0 ;
  for ( int i = 0 ;
  i < m ;
  i ++ ) {
    maxOil = Math . max ( maxOil , oil [ i ] ) ;
    costTime += length [ i ] ;
    oilLeft = oilLeft + oil [ i ] - length [ i ] ;
    while ( oilLeft < 0 ) {
      oilLeft += maxOil ;
      costTime += k ;
    }
  }
  System . out . print ( costTime ) ;
}

public static void main ( String [ ] args ) { Scanner scanner = new Scanner ( System . in ) ; int m = scanner . nextInt ( ) ; int k = scanner . nextInt ( ) ; int [ ] length = new int [ m ] ; int [ ] oil = new int [ m ] ; for ( int i = 0 ; i < m ; i ++ ) { length [ i ] = scanner . nextInt ( ) ; } for ( int i = 0 ; i < m ; i ++ ) { oil [ i ] = scanner . nextInt ( ) ; } int maxOil = 0 ; int costTime = 0 ; int oilLeft = 0 ; for ( int i = 0 ; i < m ; i ++ ) { maxOil = Math . max ( maxOil , oil [ i ] ) ; costTime += length [ i ] ; oilLeft = oilLeft + oil [ i ] - length [ i ] ; while ( oilLeft < 0 ) { oilLeft += maxOil ; costTime += k ; } } System . out . print ( costTime ) ; }
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