def ok(score, pair)
    head = N.size - 1
    count = 0
    N.each_index.inject(head) do |big, small|
        break unless big > small
        next big if big == pair || small == pair
        
        oscore = N[big] + N[small]
        if oscore > score
            count += 1
            big - 1
        else
           big
        end
    end
    count < M
end

n, M = gets.split.map(&:to_i)
N = n.times.map{ gets.to_i }
K = N.shift
N.sort!

min_pair = N.bsearch_index do |pair|
    ok(N[pair] + K, pair)
end

if min_pair.nil?
    puts -1
else
    puts N[min_pair]
end