def isOK(A):
    pre_l, pre_r = A[0]
    for i in range(1, len(A)):
        li, ri = A[i]
        if ri < pre_l:
            return False
        pre_l = max(pre_l, li)
        pre_r = ri
    return True


from itertools import permutations

N = int(input())
P = [list(map(int, input().split())) for _ in range(N)]
per = list(permutations(P))
ans = 0
for p in per:
    if isOK(p):
        ans += 1
print(ans)