#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using P = pair<int, int>;
using PL = pair<lint, lint>;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
#define ALL(a)  (a).begin(),(a).end()
constexpr int MOD = 1000000007;
constexpr lint B1 = 1532834020;
constexpr lint M1 = 2147482409;
constexpr lint B2 = 1388622299;
constexpr lint M2 = 2147478017;
constexpr int INF = 2147483647;
void yes(bool expr) {cout << (expr ? "YES" : "NO") << "\n";}
template<class T>void chmax(T &a, const T &b) { if (a<b) a=b; }
template<class T>void chmin(T &a, const T &b) { if (b<a) a=b; }
int main()
{
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  lint N, M;
  cin >> N >> M;
  vector<lint> B(M+2);
  B[0] = 0;
  REP(i, M) cin >> B[i+1];
  lint sum = 0;
  if(M > 1) {
    IREP(i, M) {
      double r = N - B[i];
      double l = N - B[i+1];
      sum += N - B[i+1];
      double W = (double)(i + sum) / (double)(M - i) + 1;
      //cout << l << " " << W << " " << r << endl;
      if(l <= W && W <= r) {
        cout << fixed << setprecision(10) << B[1] - 1 + W << endl;
        return 0;
      }
    }
  }
  cout << N - 1 << endl;
}