public static void main ( String [ ] args ) {
  FastReader fs = new FastReader ( ) ;
  int n = fs . nextInt ( ) ;
  int location = fs . nextInt ( ) ;
  boolean [ ] cities = new boolean [ n ] ;
  int criminals = 0 ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    cities [ i ] = fs . nextInt ( ) == 1 ;
  }
  if ( n == 1 ) {
    System . out . println ( cities [ 0 ] ? 1 : 0 ) ;
    return ;
  }
  if ( cities [ location - 1 ] ) {
    criminals ++ ;
  }
  int left = location - 2 ;
  int right = location ;
  while ( true ) {
    if ( left >= 0 && right < n ) {
      if ( cities [ left ] && cities [ right ] ) {
        criminals += 2 ;
      }
      left -- ;
      right ++ ;
    }
    else if ( left == - 1 && right < n ) {
      criminals += cities [ right ++ ] ? 1 : 0 ;
    }
    else if ( left >= 0 && right == n ) {
      criminals += cities [ left -- ] ? 1 : 0 ;
    }
    else {
      break ;
    }
  }
  System . out . println ( criminals ) ;
}

public static void main ( String [ ] args ) { FastReader fs = new FastReader ( ) ; int n = fs . nextInt ( ) ; int location = fs . nextInt ( ) ; boolean [ ] cities = new boolean [ n ] ; int criminals = 0 ; for ( int i = 0 ; i < n ; i ++ ) { cities [ i ] = fs . nextInt ( ) == 1 ; } if ( n == 1 ) { System . out . println ( cities [ 0 ] ? 1 : 0 ) ; return ; } if ( cities [ location - 1 ] ) { criminals ++ ; } int left = location - 2 ; int right = location ; while ( true ) { if ( left >= 0 && right < n ) { if ( cities [ left ] && cities [ right ] ) { criminals += 2 ; } left -- ; right ++ ; } else if ( left == - 1 && right < n ) { criminals += cities [ right ++ ] ? 1 : 0 ; } else if ( left >= 0 && right == n ) { criminals += cities [ left -- ] ? 1 : 0 ; } else { break ; } } System . out . println ( criminals ) ; }
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