#include<bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint=modint998244353; //1000000007;
using ll=long long;
using pp=pair<int,int>;
#define sr string 
#define vc vector
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define pb push_back
#define all(v) v.begin(),v.end()
#define pque priority_queue
#define bpc(a) __builtin_popcount(a)
int main(){
  int n,m;cin>>n>>m;
  vc<int>v(m); rep(i,m)cin>>v[i];
  if(v[0]==n){
    cout<<n-1;
    return 0;
  }
  int ac=0,wa=m-1;
  double ans=v[0]-1;
  while(wa-ac>1){
    int x=(wa+ac)/2;
    double s=0;
    for(int i=x+1;i<m;i++)s+=n-v[i];
    s/=m-(x+1); s+=(double)m/(m-x-1);
    if(s>(double)n-v[x])wa=x;
    else ac=x;
  }
  double s=0;
  for(int i=ac+1;i<m;i++)s+=n-v[i];
  s/=m-ac-1; s+=(double)m/(m-ac-1);
  ans+=s;
  cout<<fixed<<setprecision(20)<<ans;
}