# https://yukicoder.me/problems/no/801

import itertools

def main():
    N = int(input())
    lr = []
    for _ in range(N):
        L, R = map(int, input().split())
        lr.append((L, R))
    
    answer= 0
    for p in itertools.permutations([j for j in range(N)]):
        x = -1
        is_ok = True
        for i in p:
            l, r = lr[i]
            if x > r:
                is_ok = False
                break

            x = max(x, l)
        if is_ok:
            answer += 1
    print(answer)







if __name__ == "__main__":
    main()