def leftcalc(arr, start)
  res = 0
  (start - 2).downto(0) do |i|
    if arr[i] >= 2
      res += arr[i]
    elsif arr[i] == 1
      res += arr[i]
      break
    elsif arr[i] == 0
      break
    end
  end
  return res
end

def rightcalc(arr, start)
  res = 0
  start.upto(arr.size - 1) do |i|
    if arr[i] >= 2
      res += arr[i]
    elsif arr[i] == 1
      res += arr[i]
      break
    elsif arr[i] == 0
      break
    end
  end
  return res
end

N, K = gets.split.map(&:to_i)
arr = gets.split.map(&:to_i)

if K > 1 && K < N
leftsum = leftcalc(arr, K)
rightsum = rightcalc(arr, K)
elsif K == 1
  leftsum = 0
  rightsum = rightcalc(arr, K)
elsif K == N
  leftsum = leftcalc(arr, K)
  rightsum = 0
end

if arr[K - 1] >= 2
  puts arr[K - 1] + leftsum + rightsum
elsif arr[K - 1] == 1
  puts arr[K - 1] + leftsum >= rightsum ? leftsum : rightsum
elsif arr[K - 1] == 0
  puts 0
end