import itertools
N = int(input())
Q = [tuple(map(int, input().split())) for _ in range(N)]

ans = 0
for perm in itertools.permutations(range(N)):
    diff = 0
    for i in range(N):
        if diff > Q[perm[i]][1]:
            break
        else:
            diff = max(diff, Q[perm[i]][0])
    else:
        ans += 1
print(ans)