using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
class Program
{
    static string ReadLine() => Console.ReadLine().Trim();
    static int ReadInt() => int.Parse(ReadLine());
    static long ReadLong() => long.Parse(ReadLine());
    static int[] ReadIntArray() { string str = ReadLine(); return str != "" ? str.Split().Select(_ => int.Parse(_)).ToArray() : new int[0]; }
    static long[] ReadLongArray() { string str = ReadLine(); return str != "" ? str.Split().Select(_ => long.Parse(_)).ToArray() : new long[0]; }

    static (int a, int b) ReadInt2() { int[] vs = ReadIntArray(); return (a: vs[0], b: vs[1]); }
    static (int a, int b, int c) ReadInt3() { int[] vs = ReadIntArray(); return (a: vs[0], b: vs[1], c: vs[2]); }
    static (int a, int b, int c, int d) ReadInt4() { int[] vs = ReadIntArray(); return (a: vs[0], b: vs[1], c: vs[2], d: vs[3]); }
    static (long a, long b) ReadLong2() { long[] vs = ReadLongArray(); return (a: vs[0], b: vs[1]); }
    static (long a, long b, long c) ReadLong3() { long[] vs = ReadLongArray(); return (a: vs[0], b: vs[1], c: vs[2]); }
    static (long a, long b, long c, long d) ReadLong4() { long[] vs = ReadLongArray(); return (a: vs[0], b: vs[1], c: vs[2], d: vs[3]); }

    class LR
    {
        public LR(int l, int r)
        {
            L = l;
            R = r;
        }
        public int L = 0;
        public int R = 0;
    }

    static void Main()
    {
        SourceExpander.Expander.Expand();

        int N = ReadInt();

        List<LR> lrs = new List<LR>();
        int[] arr = new int[N];
        for (int i = 0; i < N; i++)
        {
            (int L, int R) = ReadInt2();
            lrs.Add(new LR(L, R));
            arr[i] = i;
        }

        int ans = 0;
        do
        {
            if (Check())
                ans++;
        }
        while (AtCoder.StlFunction.NextPermutation(arr));

        Console.WriteLine(ans);

        bool Check()
        {
            List<LR> lrs2 = new List<LR>();
            for (int i = 0; i < N; i++)
            {
                int idx = arr[i];
                lrs2.Add(lrs[idx]);
            }
            int min = int.MaxValue;
            for (int i = 0; i < N - 1; i++)
            {
                LR mae = lrs2[i];
                LR ato = lrs2[i + 1];

                if (i > 0 && ato.R < min)
                    return false;

                min = Math.Max(mae.L, ato.L);

                if (ato.R < min)
                    return false;
            }
            return true;
        }

    }
}
#region Expanded by https://github.com/kzrnm/SourceExpander
namespace AtCoder{public static class StlFunction{public struct NextPermutationEnumerator<T>:IEnumerator<T[]>,IEnumerable<T[]>where T:IComparable<T>{internal readonly IEnumerable<T>_orig;internal NextPermutationEnumerator(IEnumerable<T>orig){_orig=orig;Current=null;}public T[]Current{get;private set;}[MethodImpl(256)]public bool MoveNext(){if(Current==null){Current=_orig.ToArray();return true;}return NextPermutation(Current);}public void Reset()=>Current=null;object IEnumerator.Current=>Current;void IDisposable.Dispose(){}[MethodImpl(256)]public NextPermutationEnumerator<T>GetEnumerator()=>this;IEnumerator<T[]>IEnumerable<T[]>.GetEnumerator()=>this;IEnumerator IEnumerable.GetEnumerator()=>this;}[MethodImpl(256)]public static bool NextPermutation<T>(T[]array)where T:IComparable<T> =>NextPermutation(array.AsSpan());[MethodImpl(256)]public static bool NextPermutation<T>(Span<T>span)where T:IComparable<T>{int i;for(i=span.Length-2;i>=0;i--)if(span[i].CompareTo(span[i+1])<0)break;if(i<0)return false;int j;for(j=span.Length-1;j>=0;j--)if(span[i].CompareTo(span[j])<0)break;(span[i],span[j])=(span[j],span[i]);span.Slice(i+1,span.Length-i-1).Reverse();return true;}[MethodImpl(256)]public static NextPermutationEnumerator<T>Permutations<T>(IEnumerable<T>orig)where T:IComparable<T> =>new NextPermutationEnumerator<T>(orig);private struct DefaultComparer<T>:IComparer<T>where T:IComparable<T>{[MethodImpl(256)]public int Compare(T x,T y)=>x.CompareTo(y);}[MethodImpl(256)]public static int BinarySearch(int ok,int ng,Predicate<int>Ok){while(Math.Abs(ok-ng)>1){var m=(ok+ng)>>1;if(Ok(m))ok=m;else ng=m;}return ok;}[MethodImpl(256)]public static long BinarySearch(long ok,long ng,Predicate<long>Ok){while(Math.Abs(ok-ng)>1){var m=(ok+ng)>>1;if(Ok(m))ok=m;else ng=m;}return ok;}}}
namespace SourceExpander{public class Expander{[Conditional("EXP")]public static void Expand(string inputFilePath=null,string outputFilePath=null,bool ignoreAnyError=true){}public static string ExpandString(string inputFilePath=null,bool ignoreAnyError=true){return "";}}}
#endregion Expanded by https://github.com/kzrnm/SourceExpander