#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
#define rep(i, n) for(ll i = 0; i < n; i++)
#define rep2(i, m, n) for(ll i = m; i <= n; i++)
#define rrep(i, m, n) for(ll i = m; i >= n; i--)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define MAX(x) *max_element(all(x))
#define MIN(x) *min_element(all(x))
#define SZ(a) ((ll)(a).size())
using namespace std;
typedef long long ll;
typedef pair<int, int> P;
const ll mod = 1000000007;
const int dx[4] = { -1, 1, 0, 0 };
const int dy[4] = { 0, 0, 1, -1 };
const int inf = 1e9 + 1;
const ll INF = 1e18 + 1;
const double pi = acos(-1);

using Graph = vector<vector<int>>;
Graph G;

ll powll(ll a, ll b) {
    ll ret = 1;
    rep(i, b) ret *= a;

    return ret;
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, m; cin >> n >> m;

    if (m == 1) {
        cout << n - 1 << endl;
        return 0;
    }
    
    vector<int> b(m);
    rep(i, m) cin >> b[i];
    long double res = b[m - 1] - b[0];

    long double sum = 0.0;

    rrep(i, m - 1, 1) {
        sum += b[m - 1] - b[i];
        long double x = (long double)m / (m - i);
        long double y = (long double)sum / (m - i);
        res = min(res, x + y);
    }

    cout << setprecision(15) << res + b[0] - 1 + n - b[m - 1] << endl;

}