#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
using namespace std;

using ll = long long;
constexpr int INF = 1001001001;
constexpr int mod = 1000000007;
// constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N, M;
    cin >> N >> M;
    vector<int> B(M);
    for(int i = 0; i < M; ++i)  cin >> B[i];

    double lb = 0, ub = 1001001001, ans;
    for(int loop = 0; loop < 100; ++loop){
        double X = (lb + ub) / 2;
        vector<double> dp(M, 1e+30);
        dp[M - 1] = 0.0;
        double S = 0.0;
        for(int i = M - 2; i >= 0; --i){
            dp[i] = min(dp[i + 1] + B[i + 1] - B[i], X + 1);
            S += dp[i];
        }
        if(S / M >= X)  lb = X;
        else    ub = X;
        ans = dp[0];
    }

    cout << fixed << setprecision(10);
    cout << ans + B[0] - 1 + N - B[M - 1] << endl;

    return 0;
}