for i in 0..20
	aim_time = gets.to_i
	study_times = gets.to_i
	sum = 0
	for i in 1..study_times
		a = Array.new(2)
		a = gets.split(nil)
		s_hour = a[1].to_i - a[0].to_i
		sum = sum + s_hour
	end
	if aim_time <= sum
		puts "OK"
	else 
		puts aim_time - sum
	end
end