import itertools
n=int(input())
a=[list(map(int,input().split())) for i in range(n)]
ans=0
for i in itertools.permutations([ii for ii in range(n)],n):
	s=[a[i[j]][0] for j in range(n)]
	t=[a[i[j]][1] for j in range(n)]
	z=-1
	for j in range(n-1):
	    z=max(z,s[j])
	    if z>t[j+1]:
	        break 
	else:
		ans+=1 
print(ans)