N, M = gets.split.map(&:to_i)
A = N.times.map { gets.to_i }
B = A[1..-1].sort.reverse

def f(idx)
  x = A[0] + B[idx]
  left = 0
  right = B.size - 1

  cnt = 0

  while left < right
    if left == idx
      left += 1
      next
    elsif right == idx
      right -= 1
      next
    end

    v = B[left] + B[right]

    if v <= x
      right -= 1
    else
      cnt += 1
      left += 1
      right -= 1
    end
  end

  cnt < M
end

if not f(0)
  puts -1
  exit
end

if f(B.size - 1)
  puts B.last
  exit
end

ok = 0
ng = B.size - 1

while (ok - ng).abs >= 2
  idx = (ok + ng) / 2

  if f(idx)
    ok = idx
  else
    ng = idx
  end
end

puts B[ok]