local mfl, mce = math.floor, math.ceil
local mmi, mma = math.min, math.max
local bls, brs = bit.lshift, bit.rshift

local n, m = io.read("*n", "*n")
local a = io.read("*n")
local t = {}
n = n - 1
for i = 1, n do
  t[i] = io.read("*n")
end
table.sort(t)

local function judge(x)
  local tgt = a + t[x]
  local z = {}
  for i = n, 1, -1 do
    if i ~= x then
      table.insert(z, i)
    end
    if 2 * m <= #z then break end
  end
  for i = 1, m do
    if t[z[i]] + t[z[2 * m + 1 - i]] <= tgt then
      return true
    end
  end
  return false
end
if m * 2 == n then
  print(t[1])
elseif not judge(n) then
  print(-1)
elseif judge(1) then
  print(t[1])
else
  local min, max = 1, n
  while 1 < max - min do
    local mid = brs(min + max, 1)
    if judge(mid) then
      max = mid
    else
      min = mid
    end
  end
  print(t[max])
end