#include<bits/stdc++.h>
using namespace std;
//#define int long long
#define REP(i,m,n) for(int i=(m);i<(n);i++)
#define rep(i,n) REP(i,0,n)
#define pb push_back
#define all(a) a.begin(),a.end()
#define rall(c) (c).rbegin(),(c).rend()
#define mp make_pair
#define endl '\n'
#define vec vector<ll>
#define mat vector<vector<ll> >
#define fi first
#define se second
#define double long double
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> pll;
//typedef long double ld;
typedef complex<double> Complex;
const ll INF=1e9+7;
const ll MOD=998244353;
const ll inf=INF*INF;
const ll mod=MOD;
const ll MAX=200010;





signed main(){
    ll n,m;cin>>n>>m;
    vector<ll>b(m);
    rep(i,m)cin>>b[i];
    double ans=n-1;
    double sum=n-b[m-1];
    for(int i=m-2;i>=0;i--){
        ans=min(ans,(double)(sum+m)/((double)m-(i+1))+(double)(b[0]-1));
        sum+=n-b[i];
    }
    cout<<fixed<<setprecision(10)<<ans<<endl;
}