using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("8");
            WillReturn.Add("5 15");
            WillReturn.Add("40 110");
            WillReturn.Add("700 1400");
            WillReturn.Add("600 800");
            WillReturn.Add("900 1500");
            WillReturn.Add("1800 2300");
            WillReturn.Add("2100 2500");
            WillReturn.Add("2400 3000");
            //9
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("7");
            WillReturn.Add("12 12");
            WillReturn.Add("54 54");
            WillReturn.Add("128 128");
            WillReturn.Add("1310 1310");
            WillReturn.Add("1193 1193");
            WillReturn.Add("1806 1806");
            WillReturn.Add("2876 2876");
            //1
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    static long[] GetSplitArr(string pStr)
    {
        return (pStr == "" ? new string[0] : pStr.Split(' ')).Select(pX => long.Parse(pX)).ToArray();
    }

    struct RangeInfoDef
    {
        internal long RangeMin;
        internal long RangeMax;
    }
    static List<RangeInfoDef> mRangeInfoList = new List<RangeInfoDef>();

    static void Main()
    {
        List<string> InputList = GetInputList();

        long[] wkArr = { };
        Action<string> SplitAct = (pStr) => wkArr = GetSplitArr(pStr);

        foreach (string EachStr in InputList.Skip(1)) {
            SplitAct(EachStr);
            RangeInfoDef WillAdd;
            WillAdd.RangeMin = wkArr[0];
            WillAdd.RangeMax = wkArr[1];
            mRangeInfoList.Add(WillAdd);
        }

        var IndList = new List<int>();
        for (int I = 0; I <= mRangeInfoList.Count - 1; I++) {
            IndList.Add(I);
        }

        long Answer = 0;
        foreach (int[] EachArr in RubyPatternClass<int>.Permutation(IndList, IndList.Count)) {

            bool IsOK=true;
            long RunMax = long.MinValue;
            foreach (int EachInd in EachArr) {
                long RangeMin = mRangeInfoList[EachInd].RangeMin;
                long RangeMax = mRangeInfoList[EachInd].RangeMax;

                if (RunMax <= RangeMin) {
                    RunMax = RangeMin;
                    continue;
                }
                if (RangeMin <= RunMax && RunMax <= RangeMax) {
                    RunMax = RunMax;
                    continue;
                }
                IsOK = false;
                break;
            }
            if (IsOK) Answer++;
        }
        Console.WriteLine(Answer);
    }
}

#region RubyPatternClass
// Rubyの場合の数
internal static class RubyPatternClass<Type>
{
    // 順列を返す
    private struct JyoutaiDef_Permutation
    {
        internal List<int> SelectedIndList;
    }
    internal static IEnumerable<Type[]> Permutation(IEnumerable<Type> pEnum, int pR)
    {
        if (pR == 0) yield break;
        Type[] pArr = pEnum.ToArray();
        if (pArr.Length < pR) yield break;

        var Stk = new Stack<JyoutaiDef_Permutation>();
        JyoutaiDef_Permutation WillPush;
        for (int I = pArr.GetUpperBound(0); 0 <= I; I--) {
            WillPush.SelectedIndList = new List<int>() { I };
            Stk.Push(WillPush);
        }

        while (Stk.Count > 0) {
            JyoutaiDef_Permutation Popped = Stk.Pop();

            // クリア判定
            if (Popped.SelectedIndList.Count == pR) {
                var WillReturn = new List<Type>();
                Popped.SelectedIndList.ForEach(X => WillReturn.Add(pArr[X]));
                yield return WillReturn.ToArray();
                continue;
            }

            for (int I = pArr.GetUpperBound(0); 0 <= I; I--) {
                if (Popped.SelectedIndList.Contains(I)) continue;
                WillPush.SelectedIndList = new List<int>(Popped.SelectedIndList) { I };
                Stk.Push(WillPush);
            }
        }
    }
}
#endregion