list1 = [1,3,4,5,9,13]
list2 = [2,10,12]

out = []

while len(list1) and len(list2):
	if list1[0] < list2[0]:
		out += [list1[0]]
		list1 = list1[1:]
	else:
		out += [list2[0]]
		list2 = list2[1:]

out += list1 + list2

print out