#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

int main(){
    int N, M; cin >> N >> M;

    vector<bool> a(N, false);
    vector<int> B(M);
    rep(i, M){
        cin >> B[i]; B[i]--;
        a[B[i]] = true;
    }

    double L = 0, R = N+1;
    vector<double> dp(N);

    rep(t, 100){
        double m = (L+R)/2;

        dp[N-1] = 0;

        rep3(i, N-2, 0){
            if(a[i]) dp[i] = 1+min(m, dp[i+1]);
            else dp[i] = 1+dp[i+1];
        }

        double S = 0;
        rep(i, M) S += dp[B[i]];
        S /= M;

        if(S > m) L = m;
        else R = m;
    }

    cout << dp[0] << '\n';
}