using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    static long[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var map = NArr(n);
        var ans = 0;
        var INF = long.MaxValue;
        Permutate(n, list =>
        {
            var min = 0L;
            for (var i = 0; i < n; ++i)
            {
                min = Math.Max(min, map[list[i]][0]);
                if (min > map[list[i]][1]) min = INF;
            }
            if (min < INF) ++ans;
        });
        WriteLine(ans);
    }
    static void Permutate(int n, Action<int[]> action)
    {
        var seed = new int[n];
        for (var i = 0; i < n; ++i) seed[i] = i;
        var used = new bool[n];
        var perm = new int[n];
        Permutate(0, seed, perm, used, action);
    }
    static void Permutate(int pos, int[] seed, int[] perm, bool[] used, Action<int[]> action)
    {
        if (pos == used.Length) action(perm);
        for (var i = 0; i < used.Length; ++i)
        {
            if (used[i]) continue;
            perm[pos] = seed[i];
            used[i] = true;
            Permutate(pos + 1, seed, perm, used, action);
            used[i] = false;
        }
    }
}