public static void main ( String [ ] args ) {
  Locale . setDefault ( Locale . US ) ;
  Scanner in = new Scanner ( System . in ) ;
  int N = in . nextInt ( ) ;
  int K = in . nextInt ( ) ;
  double sum = 0 ;
  for ( int n = 1 ;
  n <= N ;
  n ++ ) {
    int a = in . nextInt ( ) ;
    double count = Math . min ( Math . min ( n , N + 1 - n ) , Math . min ( K , N - K + 1 ) ) ;
    sum += a * count ;
  }
  sum /= N - K + 1 ;
  System . out . println ( sum ) ;
}

public static void main ( String [ ] args ) { Locale . setDefault ( Locale . US ) ; Scanner in = new Scanner ( System . in ) ; int N = in . nextInt ( ) ; int K = in . nextInt ( ) ; double sum = 0 ; for ( int n = 1 ; n <= N ; n ++ ) { int a = in . nextInt ( ) ; double count = Math . min ( Math . min ( n , N + 1 - n ) , Math . min ( K , N - K + 1 ) ) ; sum += a * count ; } sum /= N - K + 1 ; System . out . println ( sum ) ; }
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