from itertools import permutations

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

ans = 0
for P in permutations(range(N)):
    difficult = list(map(lambda p: problem_difficult_range[p], P))
    for diff in difficult[0]:
        is_ok = True
        # problems_difficult = []
        prv_difficult = diff
        # problems_difficult.append(prv_difficult)
        for suf in difficult[1:]:
            if prv_difficult <= suf[0]:
                prv_difficult = suf[0]
                # problems_difficult.append(prv_difficult)
            elif prv_difficult <= suf[1]:
                prv_difficult = suf[1]
                # problems_difficult.append(prv_difficult)
            else:
                is_ok = False
                break

        if is_ok:
            ans += 1
            # print(problems_difficult)
            break

print(ans)