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

left_cnt = 0
right_cnt = 0

(K - 2).downto(0) do |k|
  left_cnt += A[k]

  break if A[k] <= 1
end

K.upto(N - 1) do |k|
  right_cnt += A[k]

  break if A[k] <= 1
end

if A[K - 1] == 0
  puts 0
elsif A[K - 1] == 1
  puts [left_cnt, right_cnt].max + 1
else
  puts left_cnt + right_cnt + A[K - 1]
end