from itertools import permutations

N = int(input())
LR = [list(map(int, input().split())) for _ in range(N)]

ans = 0
for perm in permutations(range(N)):
    diff = 0
    for p in perm:
        L, R = LR[p]
        if R < diff:
            break
        diff = max(diff, L)
    else:
        ans += 1

print(ans)