#include <iostream>
#include <iomanip>
#include <vector>

using namespace std;
using ldouble = long double;

void solve() {
    int n, m;
    cin >> n >> m;

    vector<ldouble> ds(m);
    for (auto& d : ds) {
        int b;
        cin >> b;
        d = n - b;
    }

    ldouble best = ds[0];  // warpless
    {
        ldouble sum = 0;
        for (int k = m - 1; k >= 0; --k) {
            sum += ds[k];
            best = min(best, (m + sum) / (m - k));
        }
    }

    cout << (n - 1 - ds[0] + best) << "\n";
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);

    solve();

    return 0;
}