public static void main ( String [ ] args ) {
  Scanner sc = new Scanner ( System . in ) ;
  int N = sc . nextInt ( ) ;
  int K = sc . nextInt ( ) ;
  int [ ] a = new int [ N ] ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    a [ i ] = sc . nextInt ( ) ;
  }
  int [ ] cum = new int [ N + 1 ] ;
  for ( int i = 0 ;
  i <= N - K ;
  i ++ ) {
    cum [ i ] ++ ;
    cum [ i + K ] -- ;
  }
  for ( int i = 1 ;
  i <= N ;
  i ++ ) {
    cum [ i ] += cum [ i - 1 ] ;
  }
  long ans = 0 ;
  for ( int i = 0 ;
  i < N ;
  i ++ ) {
    ans += ( long ) a [ i ] * cum [ i ] ;
  }
  out . println ( ans ) ;
}

public static void main ( String [ ] args ) { Scanner sc = new Scanner ( System . in ) ; int N = sc . nextInt ( ) ; int K = sc . nextInt ( ) ; int [ ] a = new int [ N ] ; for ( int i = 0 ; i < N ; i ++ ) { a [ i ] = sc . nextInt ( ) ; } int [ ] cum = new int [ N + 1 ] ; for ( int i = 0 ; i <= N - K ; i ++ ) { cum [ i ] ++ ; cum [ i + K ] -- ; } for ( int i = 1 ; i <= N ; i ++ ) { cum [ i ] += cum [ i - 1 ] ; } long ans = 0 ; for ( int i = 0 ; i < N ; i ++ ) { ans += ( long ) a [ i ] * cum [ i ] ; } out . println ( ans ) ; }
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