#include <iostream>
#include <cstdio>
#include <string>
#include <algorithm>
#include <utility>
#include <cmath>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <tuple>
#include <numeric>
#include <functional>
using namespace std;
typedef long long ll;
typedef vector<ll> vl;
typedef vector<vector<ll>> vvl;
typedef pair<ll, ll> P;
#define rep(i, n) for(ll i = 0; i < n; i++)
#define exrep(i, a, b) for(ll i = a; i <= b; i++)
#define out(x) cout << x << endl
#define exout(x) printf("%.10f\n", x)
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define pb push_back
#define re0 return 0
const ll mod = 1000000007;
const ll INF = 1e16;
  
int main() {
    ll n, m;
    cin >> n >> m;

    vl b(m);
    rep(i, m) {
        cin >> b[i];
    }
    vl c(m+1);
    rep(i, m) {
        c[i+1] = c[i] + b[m - 1 - i];
    }

    double ans = n-1;
    exrep(i, 1, m) {
        double x = b[0] - 1 + ((double)m / (double)i) + n - ((double)c[i] / (double)i);
        chmin(ans, x);
    }

    exout(ans);
    re0;
}