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)
  t = []
  s = []
  as.each_with_index do |b, idx|
    next if idx == ignore_index
    s << b
    if s.size == 2
      t << s.inject(:+)
      s = []
    end
    break if t.size == m
  end
  a >= t[-1]
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
    100.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