/* -*- coding: utf-8 -*-
 *
 * 1478.cc:  No.1478 Simple Sugoroku - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_M = 100000;
const int CNT = 50;

/* typedef */

/* global variables */

int bs[MAX_M + 1];
double es[MAX_M + 1];

/* subroutines */

double check(int n, int m, double s) {
  es[m] = 0.0;
  double d = 1 + s / m, sum = 0.0;
  for (int i = m - 1; i >= 0; i--) {
    es[i] = min(d, es[i + 1] + bs[i + 1] - bs[i]);
    sum += es[i];
  }
  return sum;
}

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  for (int i = 0; i < m; i++) scanf("%d", bs + i);
  bs[m] = n;

  /*
    e_i=min(1.0+sum ej/m, di+e_{i+1})
   */

  double s0 = 0.0, s1 = (double)n * m;
  for (int cnt = 0; cnt < CNT; cnt++) {
    double s = (s0 + s1) / 2;
    if (check(n, m, s) <= s) s1 = s;
    else s0 = s;
  }

  check(n, m, s1);
  printf("%.10lf\n", es[0] + bs[0] - 1);
  return 0;
}