#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define chmin(x,y) x = min((x),(y))
#define chmax(x,y) x = max((x),(y))
#define popcount(x) __builtin_popcount(x)
using namespace std;
using ll = long long ;
using P = pair<int,int> ;
using pll = pair<long long,long long>;
const int INF = 1e9;
const long long LINF = 1e17;
const int MOD = 1000000007;
//const int MOD = 998244353;
const double PI = 3.14159265358979323846;

int main(){
    ll n;
    cin >> n;
    ll m;
    cin >> m;
    vector<ll> b(m);
    rep(i,m) cin >> b[i];

    if(m==1){
        cout << n-1LL << endl;
        return 0;
    }

    double ans = b[m-1]-b[0];
    ll sum = 0;
    for(int i=m-1;i>=0;i--){
        sum += b[m-1]-b[i];
        double x = (m+sum) / (double)(m-i);
        chmin(ans,x);
    }
    ans += b[0]-1LL+n-b[m-1];
    cout <<  setprecision(20) << ans << endl;

    return 0;
}