#include<bits/stdc++.h>
using namespace std;
#define sync ios_base::sync_with_stdio(false), cin.tie(NULL)
#define F first
#define S second
#define eb emplace_back
#define pb emplace_back
#define mt make_tuple
#define gcd __gcd
// Input
#define in(a) scanf("%d",&a)
#define in2(a,b) scanf("%d%d",&a,&b)
#define in3(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define llin(a) cin >> a
#define inl(a) scanf("%lld",&a)
#define read(v,i,n) for(i=0;i<n;i++)in(v[i])
#define twod(mat,i,j,n,m) rep(i,n){rep(j,m)in(mat[i][j]);}
#define sc(ch) scanf("%c",&ch)
#define sstr(str) scanf("%s",str)
// Output
#define pr(a) printf("%d ",a)
#define pr2(a,b) printf("%d %d\n",a,b);
#define pr3(a,b,c) printf("%d %d %d\n",a,b,c);
#define out(a) printf("%d\n",a)
#define outl(a) printf("%lld\n",a)
#define llpr(a) cout << a << " "
#define llout(a) cout << a << "\n"
#define yes printf("YES\n")
#define no printf("NO\n")
#define lin printf("\n")
// Iterator
#define lp(i,a,b) for(i=a;i<b;i++)
#define rep(i,n) for(i=0;i<n;i++)
#define all(vec) vec.begin(),vec.end()
#define lower(v,k) lower_bound(v.begin(),v.end(),k)-v.begin()
#define upper(v,k) upper_bound(v.begin(),v.end(),k)-v.begin()
#define tf(mytuple) get<0>(mytuple)
#define ts(mytuple) get<1>(mytuple)
#define tt(mytuple) get<2>(mytuple)
// Debug
#define dbg(v,i,n) for(i=0;i<n;i++)pr(v[i]); lin
#define what(x) cerr << #x << " : " << x << "\n";
#define ck printf("continue\n")
#define debug(args...) { string _s = #args; replace(_s.begin(), _s.end(), ',', ' '); stringstream _ss(_s); istream_iterator<string> _it(_ss); err(_it, args); }
void err(istream_iterator<string> it) {}
template<typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args)
{
	cerr << *it << " = " << a << "\n";
	err(++it, args...);
}
// Data Type
#define ll long long int
#define ii pair<int,int>
#define pli pair<ll,int>
#define state tuple<int,int,int>
#define vi vector<int>
#define vii vector<pair<int,int> >
#define vvi vector<vector<int> >
#define viii vector<pair<pair<int,int>,int> >
#define vvii vector<vector<pair<int,int> > >
#define M 1000000007
#define Z 505  //check the limits, man
#define N 605
const ll INF = 1e10;
double dp[N],v[N];
int edge[N][N];
int deg[N];
void build(int n)
{
    int d,i,j;
    dp[n-1]=0;
    v[0]=1;
    for(i=n-2;i>=0;i--)
    {
        d=0;
        for(j=i+1;j<n;j++)
            if(edge[i][j])dp[i]+=dp[j],d++;
        dp[i]/=d;
        dp[i]+=1;
    }
    rep(i,n)
    {
        lp(j,i+1,n)
            if(edge[i][j])v[j]+=(v[i]/deg[i]);
    }
    return ;
}
double ar[N];
double fun(int s,int d,int n)
{
    int i,j;
    if(deg[s]==1)
        return (double)M;
    double val=dp[s]-1;
    val=val*deg[s]-dp[d];
    val=val/(deg[s]-1);
    val+=1;

    return (val-dp[s])*v[s];
}
int main()
{
    int n,m,i,j,a,b;
    in2(n,m);
    while(m--)
    {
        in2(a,b);
        a--,b--;
        edge[a][b]=1;
        deg[a]++;
    }
    cout << setprecision(12) << fixed;
    build(n);
    double ans=dp[0];
    double temp;
    rep(i,n-1)
    {
        temp=-1,a=0;
        for(j=i+1;j<n;j++)
        {
            if(edge[i][j] && dp[j]>temp)
                temp=dp[j],a=j;
        }
        ans=min(ans,dp[0]+fun(i,a,n));
    }
    llout(ans);
}









Measured memory_bytes: 1664