#include <bits/stdc++.h>
using namespace std;

struct iofast_t {
    iofast_t() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr); cout.tie(nullptr);
    }
} iofast;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return std::vector(arg, init);
    else return std::vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

template <typename T, typename Compare = less<T>>
T &chmin(T &l, T r, Compare &&f = less<T>()) { return l = min(l, r, f); }
template <typename T, typename Compare = less<T>>
T &chmax(T &l, T r, Compare &&f = less<T>()) { return l = max(l, r, f); }

int main() {
    cout << fixed << setprecision(15);

    int n, m; cin >> n >> m;
    auto b = vec<int>(uns, m);
    for (auto &e : b) cin >> e;

    auto acc = vec<int64_t>(uns, m + 1);
    acc[0] = 0;

    for (int i = 0; i < m; ++i) {
        acc[i + 1] = acc[i] + (n - b[i]);
    }

    double ans = n - 1;
    for (int i = 1; i <= m; ++i) {
        double sub = (b.front() - 1) + (m / double(i)) + (1.0 / i) * (acc[m] - acc[m - i]);
        ans = fmin(ans, sub);
    }

    cout << ans << endl;
}