n = gets.to_i
l = []
r = []
n.times do
    x, y = gets.split.map(&:to_i)
    l << x
    r << y
end

idx = []
(0..n - 1).each do |i|
    idx << i
end

ans = 0
idx.permutation do |id|
    ok = true
    x = 0
    (0..n - 1).each do |i|
        if r[id[i]] < x then
            ok = false
            break
        end
        if x < l[id[i]] then
            x = l[id[i]]
        end
    end
    if ok then
        ans += 1
    end
end

puts ans