n, m = gets.chomp.split.map(&:to_i)
a = gets.to_i
as = (2..n).map { gets.to_i }.sort.reverse

def ok(a, m, as, ignore_index)
  k = 0
  left = 0
  right = as.size - 1
  left += 1 if left == ignore_index
  right -= 1 if right == ignore_index
  while left < right
    if as[left] + as[right] > a
      k += 1
      left += 1
      right -= 1
    else
      right -= 1
    end
    left += 1 if left == ignore_index
    right -= 1 if right == ignore_index
  end
  # p ['ok-check', [a,m,as,ignore_index], k, (k <= m)]
  k < m
end

ans = -1
left = 0
right = n - 2

if ok(a + as[left], m, as, left)
  if ok(a + as[right], m, as, right)
    p as[right]
  else
    # bisearch
    30.times do
      mid = (left + right) / 2
      if ok(a + as[mid], m, as, mid)
        left = mid
      else
        right = mid
      end
    end
    p as[left]
  end
else
  p -1
end