from itertools import permutations

def main():
    N = int(input())
    P = []
    for _ in range(N):
        l, r = map(int, input().split())
        P.append((l, r))

    ans = 0
    for perm in permutations(range(N)):
        mn = 0
        isok = True
        for p in perm:
            if mn > P[p][1]:
                isok = False
                break
            mn = max(mn, P[p][0])

        if isok:
            ans += 1
    
    print(ans)
    return

main()