local n, k = io.read("*n", "*n")
local a = {}
for i = 1, n do
  a[i] = io.read("*n")
end
if a[k] == 0 then
  print(0)
else
  local left = k
  for i = k - 1, 1, -1 do
    if a[i] == 0 then
      break
    else
      left = i
      if a[i] == 1 then
        break
      end
    end
  end
  local right = k
  for i = k + 1, n do
    if a[i] == 0 then
      break
    else
      right = i
      if a[i] == 1 then
        break
      end
    end
  end
  local leftsum = 0
  for i = left, k - 1 do
    leftsum = leftsum + a[i]
  end
  local rightsum = 0
  for i = k + 1, right do
    rightsum = rightsum + a[i]
  end
  if a[k] == 1 then
    print(a[k] + math.max(leftsum, rightsum))
  else
    print(a[k] + leftsum + rightsum)
  end
end