#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef pair<ll,ll> P;
typedef vector<ll> VI;
typedef vector<VI> VVI;
#define REP(i,n) for(int i=0;i<(n);i++)
#define ALL(v) v.begin(),v.end()
constexpr ll MOD=998244353;
constexpr ll INF=2e18;

int main(){
    ll n, m; cin >> n >> m;
    VI b(m); REP(i,m) cin >> b[i];
    reverse(ALL(b));
    double ans=INF;
    ll sum=0;
    REP(i,m){
        sum+=n-b[i];
        ans=min(ans,(double)sum/(i+1)+(double)m/(i+1));
    }
    printf("%.10f\n",min(ans+b[m-1]-1,(double)n-1));
    return 0;
}