from itertools import permutations
N = int(input())
L = []
R = []
for _ in range(N):
    l,r = map(int,input().split())
    L.append(l)
    R.append(r)
ans = 0
for perm in permutations(range(N)):
    flag = True
    low = L[perm[0]]
    for i in range(N-1):
        if R[perm[i+1]]>=low:
            low = max(low,L[perm[i+1]])
            continue
        flag = False
        break
    if flag:
        ans += 1
print(ans)