/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2021.04.16 22:27:32
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    int n,m;cin >> n >> m;
    vector<int> a(m);
    double plus = 0;
    for (int i = 0; i < m; i++) {
        cin >> a[i];
        if (i == 0) plus += a[i]-1;
        else if (i == m-1) plus += n-a[i];
    }
    double left = 0, right = 1000000000;
    double ans;
    int loop_cnt = 50;
    for (int t = 0; t < loop_cnt; t++) {
        double mid = (left+right)/2;
        vector<double> dp(m,0);
        double sum = 0;
        for (int i = m-2; i >= 0; i--) {
            dp[i] = min(dp[i+1]+a[i+1]-a[i],mid+1);
            sum += dp[i];
        }
        if (sum/m >= mid) {
            left = mid;
        }
        else {
            right = mid;
        }
        if (t == loop_cnt-1) ans = dp[0];
    }
    printf("%.10f\n",ans+plus);
    return 0;
}