from itertools import permutations

N = int(input())
LR = [[int(x) for x in input().split()] for _ in range(N)]

ans = 0
for p in permutations(range(N)):
    ok = True
    d = 0
    for i in range(N):
        l, r = LR[p[i]]
        if d <= r:
            d = max(d, l)
        else:
            ok = False
            break
    ans += ok

print(ans)