from itertools import permutations

N=int(input())
mem=[]
for i in range(N):
    L, R = (int(x) for x in input().split())
    mem.append([L, R])

ans=0
for i in permutations(mem):
    ok=True
    S=[]
    for j in range(1, N):
        if i[j-1][0] <= i[j][1]:
            continue
        else:
            ok = False
    
    if ok:
        idx = 1
        x = 1
        while x <= 3854 and idx <= N-1:
            if i[idx-1][0] <= x <= i[idx][1]:
                S.append(x)
                idx += 1
            else:
            	x += 1
        
        S.append(i[-1][1])

        ok2 = True
        if len(S) < N:
            ok2 = False
            continue

        for i in range(1, N):
            if S[i-1] > S[i]:
                ok2 = False
        
        if ok2:
            ans += 1
print(ans)