#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#include <list>
#include <atcoder/all>
#define popcount __builtin_popcount
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef pair<int, int> P;

int main()
{
    ll n; cin>>n;
    int m; cin>>m;
    double ans=n-1;
    ll b[100010];
    for(int i=0; i<m; i++) cin>>b[i];
    double s=0;
    for(int i=m-1; i>=0; i--){
        s+=n-b[i];
        ans=min(ans, (s+m)/(m-i)+b[0]-1);
    }
    printf("%.6lf\n", ans);
    return 0;
}