STDOUT.sync = true
N = gets.to_i

(N / 2).times do |i|
    x = i
    y = N - 1 - i
    puts "? #{x} #{y}"
    z = gets.to_i
    if z == x
        puts "! #{y}"
        exit
    elsif z == y
        puts "! #{x}"
        exit
    end
end
puts "! #{N / 2}"