using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
static class Program{
	const int mod=(int)1e9+7;
	static List<int>[] li;
	static bool[] b;
	static double[] h;
	static void Main(){
		Sc sc=new Sc();
		int[] s=sc.Ia;
		int n=s[0],m=s[1];
		li=new List<int>[n+1];
		b=new bool[n+1];
		h=new double[n+1];
		for(int i=1;i<=n;i++){li[i]=new List<int>();}
		for(int i=0;i<s[1];i++){
			int[] e=sc.Ia;
			li[e[0]].Add(e[1]);
		}
		var li2=new List<Tuple<int,double>>();
		for(int i = n-1;i>0;i--) {
			if(li[i].Count==1){
				h[i]=h[li[i][0]]+1;
			}
			else{
				double d=0,p=0;
				for(int j = 0;j<li[i].Count;j++) {
					d+=h[li[i][j]]+1;
					p=Max(p,h[li[i][j]]+1);
				}
				li2.Add(Tuple.Create(i,p));
				h[i]=d/(li[i].Count);
			}
		}
		double ans=h[1];
		for(int k = 0;k<li2.Count;k++) {
			h=new double[n+1];
			for(int i = n-1;i>0;i--) {
				if(li[i].Count==1){
					h[i]=h[li[i][0]]+1;
				}
				else{
					double d=0;
					for(int j = 0;j<li[i].Count;j++) {
						d+=h[li[i][j]]+1;
					}
					if(i==li2[k].Item1){
						d-=li2[k].Item2;
						h[i]=d/(li[i].Count-1);
					}
					else{h[i]=d/(li[i].Count);}
				}
			}
			ans=Min(ans,h[1]);
		}
		Console.WriteLine("{0}",ans);
	}
}

public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),int.Parse);}
	public int[] Ia3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(int a){return Array.ConvertAll((a.ToString()+" "+Console.ReadLine()).Split(),long.Parse);}
	public long[] La3(bool a,int b,bool c,int d){return Array.ConvertAll(((a?b.ToString()+" ":"")+Console.ReadLine()+(c?" "+d.ToString():"")).Split(),long.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
}

Measured memory_bytes: 20016