#include <bits/stdc++.h>
#include <unistd.h>
 
using namespace std;
 
typedef long long ll;
typedef pair<ll,ll> P;
 
long long int INF = 3e18;
double Pi = 3.1415926535897932384626;
 
vector<ll> G[550010];
//vector<P> tree[500010];
priority_queue <ll> pql;
priority_queue <P> pqp;
//big priority queue
priority_queue <ll,vector<ll>,greater<ll> > pqls;
priority_queue <P,vector<P>,greater<P> > pqps;
//small priority queue
//top pop
 
int dx[8]={1,0,-1,0,1,1,-1,-1};
int dy[8]={0,1,0,-1,1,-1,-1,1};
char dir[] = "DRUL";
//ll bit[500005];
//↓,→,↑,←
 
 
#define p(x) cout<<x<<"\n";
#define el cout<<endl;
#define pe(x) cout<<(x)<<" ";
#define ps(x) cout<<fixed<<setprecision(25)<<x<<endl;
#define pu(x) cout<<(x);
#define pb push_back
#define lb lower_bound
#define ub upper_bound
#define CLEAR(a) a = decltype(a)(); 
 
 
ll mod = 998244353;
 
ll rui(ll number1,ll number2){
 
    if(number2 == 0){
        return 1;
    }else{
        ll number3 = rui(number1,number2 / 2);
        number3 *= number3;
        number3 %= mod;
        if(number2%2==1){
            number3 *= number1;
            number3 %= mod;
        }
        return number3;
    }
}
ll gcd(ll number1,ll number2){
 
    if(number1 > number2){
        swap(number1,number2);
    }
    if(number1 == 0 || number1 == number2){
        return number2;
    }else{
        return gcd(number2 % number1,number1);
    }
}
void YES(bool condition){
 
    if(condition){
        p("YES");
    }else{
        p("NO");
    }
    return;
}
void Yes(bool condition){
 
    if(condition){
        p("Yes");
    }else{
        p("No");
    }
    return;
}
 

ll fact[800005],rfact[800005];
 
void c3_init(){
    fact[0] = rfact[0] = 1;
    for(ll i=1; i<=800000; i++){
        fact[i] = (fact[i-1]*i) % mod;
    }
    rfact[800000] = rui(fact[800000],mod - 2);
    for(ll i=800000; i>=1; i--){
       rfact[i-1] = rfact[i] * i;
       rfact[i-1] %= mod;
    }
    return;}
ll c3(ll n,ll r){
    return (((fact[n] * rfact[r]) % mod ) * rfact[n-r]) % mod;}

 
ll n,m,num,sum,a,b,c,d,e,g,h,w,i,j,q,r,l;
ll k,ans;
ll x[500005],y[500005],z[500005];
P table[500005];

double dp[604];

char s[500005];

int main(){
    cin >> n >> m;
    for(int i=0;i<m;i++){
        cin >> a >> b;
        table[i].first = a;
        table[i].second = b;
    }
    sort(table,table+m);
    reverse(table,table+m);
    ll lastchecked = n;
    for(int i=0;i<m;i++){
        a = table[i].first;
        b = table[i].second;
        x[i] = a;
        y[i] = b;
        z[a] ++;
        G[a].pb(b);
    }
    x[m] = -1;
    double ans = INF;
    for(int i=1;i<=n+1;i++){
        // which node to delete
        ll deletenode = i;
        if(deletenode == n + 1){
            deletenode = -1;
        }
        if(deletenode != -1){
            if(z[deletenode] <= 1)continue;
        }
        dp[n] = 0;
        for(int j=n-1;j>=1;j--){
            double val = 0;
            ll cnt = 0;
            double valmax = 0;
            for(int k=0;k<G[j].size();k++){
                ll nv = G[j][k];
                val += dp[nv];
                valmax = max(valmax, dp[nv]);
                cnt ++;
            }
            if(deletenode == j){
                cnt -= 1;
                val -= valmax;
            }
            val /= cnt;
            dp[j] = val + 1;
        }
        ans = min(ans, dp[1]);
        //pe("deleted node: ");p(deletenode);
        //pe("ans ");ps(dp[1]);
    }
    ps(ans);
    return 0;
}











Measured memory_bytes: 32896