N, K = gets.split.map(&:to_i)
A = gets.split.map(&:to_i)
l = K - 2; r = K
l -= 1 while l > 0 && A[l] > 1
r += 1 while r < N - 1 && A[r] > 1

if A[K - 1] == 0
  puts 0
elsif A[K - 1] == 1
  puts [A[l .. K - 1].sum, A[K - 1 .. r].sum].max
else
  puts A[l .. r].sum
end