array = []
while true
	line = gets
	break if line.nil?
	a,b = line.split(' ').map(&:to_i)
	array[a] = array[a] ? array[a] + 1 : 1
	array[b] = array[b] ? array[b] + 1 : 1
	if a==0
		i = 3
		status = array[1]%2==1 && array[2]==1 ? 'OK' : 'NG'
		#p "now #{status}, #{array[1]} #{array[2]}"
		while array[i]
			if array[i]%2!=0
				status = 'NG'
				break
			end
			i = i+1
		end
		puts status
		array = []
	end
end