#include <bits/stdc++.h>
using namespace std;
//#include <atcoder/all>
//using namespace atcoder;
using ll=long long;
using Graph=vector<vector<int>>;
#define MAX 200003
#define MOD 998244353
#define INF 1000000000

int main(){
  ll N;
  int M;
  cin>>N>>M;
  vector<ll> B(M);
  for(int i=0;i<M;i++){
    cin>>B[i];
  }

  double ans=(double)(N-1);
  ll sum=0;
  for(int i=M-1;i>0;i--){
    sum+=(N-B[i]);
    double tmp=(double)(B[0]-1)+(double)M/(double)(M-i)+(double)sum/(double)(M-i);
    ans=min<double>(ans,tmp);
  }

  printf("%.9lf\n",ans);
}