public static void main ( String [ ] args ) {
  int n = scanner . nextInt ( ) , k = scanner . nextInt ( ) ;
  System . out . println ( java . util . stream . LongStream . range ( 0 , n ) . map ( i -> scanner . nextInt ( ) * Math . min ( Math . min ( Math . min ( i + 1 , n - i ) , k ) , n - k + 1 ) ) . sum ( ) ) ;
}

public static void main ( String [ ] args ) { int n = scanner . nextInt ( ) , k = scanner . nextInt ( ) ; System . out . println ( java . util . stream . LongStream . range ( 0 , n ) . map ( i -> scanner . nextInt ( ) * Math . min ( Math . min ( Math . min ( i + 1 , n - i ) , k ) , n - k + 1 ) ) . sum ( ) ) ; }
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