#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
using ll=long long;
using ull=unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)

ll N;
ll M;
vector<ll> A;

int main(){
  scanf("%lld%lld",&N,&M);
  A.resize(M);
  rep(i,M){ int a; scanf("%d",&a); A[i]=a; }
  double ans = N-1;
  double sum = 0.0;
  rep(i,M) sum += N-A[i];
  rep(i,M-1){
    sum -= N-A[i];
    ans = min(ans, (double)M / (M-i-1) + sum / (M-i-1) + (A[0]-1));
  }
  printf("%.10f\n",ans);
  return 0;
}