N = int(input())
rng = []
for i in range(N):
    l, r = map(int, input().split())
    rng.append((l, r))

import itertools
ans = 0
for t in itertools.permutations(range(N)):
    inow = t[0]
    rnow = rng[inow][0]
    ok = True
    for i in range(1, N):
        inow = t[i]
        nowmin = rng[inow][0]
        nowmax = rng[inow][1]
        if nowmax < rnow:
            ok = False
            break
        else:
            rnow = max(rnow, nowmin)
    if ok == True:
        ans += 1
print(ans)