from itertools import permutations

N=int(input())
mem=[]
for _ in range(N):
    L, R = map(int, input().split())
    mem.append([L, R])

ans=0
for perm in permutations(mem):
    idx = 0
    x = 1
    S = []
    while x <= 3854 and idx < N:
        if perm[idx][0] <= x <= perm[idx][1]:
            S.append(x)
            idx += 1
        x += 1

    if len(S) == N:  # 全部埋まった
        ok = True
        for j in range(1, N):
            if S[j-1] > S[j]:
                ok = False
                break
        if ok:
            ans += 1

print(ans)