def rank(score, pair, players)
    big = players.size - 1
    count = 0
    players.each_index do |small|
        break if small >= big
        
        if big == pair 
            big -= 1
            redo
        elsif small == pair
            next
        end
        
        oscore = players[big] + players[small]
        if oscore > score
            count += 1
            big -= 1
        end
    end
    count
end

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

min_pair = (0 .. n - 2).bsearch do |i|
    rank(K + N[i], i, N) < M
end

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