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)):
    curr = 0
    for p in perm:
        curr = max(curr + 1, L[p])
        
        if curr > R[p]:
            break
    else:
        ans += 1

print(ans)