from itertools import permutations
N = int(input())
P = [tuple(map(int, input().split())) for _ in range(N)]
ans = 0
for p in permutations(P):
    ng = 0
    m = p[0][0]
    for i in range(N-1):
        if m > p[-~i][1]:
            ng |= 1
        m = max(m, p[-~i][0])
    ans += 1-ng
print(ans)