n = int(input())

l = [list(map(int,input().split())) for _ in range(n)]

p = [[]]

ans = 0

for a in range(n):
    t = len(p)
    for i in range(t):
        for j in range(n):
            if not j in p[i]:
                p.append(p[i]+[j])

    for k in range(t):
        p.pop(0)
    
def check(p,pos,d):
    global ans

    if pos == n:
        ans += 1
    else:
        if d > l[p[pos]][1]:
            None
        elif d < l[p[pos]][0]:
            check(p,pos+1,l[p[pos]][0])
        else:
            check(p,pos+1,d)

for i in range(len(p)):
    check(p[i],0,0)

print(ans)