#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++(i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++(i))
#define REP_R(i, n) for (int i = (int)(n)-1; (i) >= 0; --(i))
#define REP3R(i, m, n) for (int i = (int)(n)-1; (i) >= (int)(m); --(i))
#define ALL(x) ::std::begin(x), ::std::end(x)
using namespace std;

double solve(int64_t n, int m, const vector<int64_t>& b) {
    vector<int64_t> c(m + 1);
    partial_sum(ALL(b), c.begin() + 1);
    double ans = n - 1;
    REP (i, m) {
        ans = min(ans, b[0] + static_cast<double>(m) / (m - i) + n - static_cast<double>(c[m] - c[i]) / (m - i) - 1);
    }
    return ans;
}

// generated by oj-template v4.7.2 (https://github.com/online-judge-tools/template-generator)
int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    constexpr char endl = '\n';
    int N, M; cin >> N >> M;
    vector<int64_t> B(M);
    REP (i, M) { cin >> B[i]; }
    auto ans = solve(N, M, B);
    cout << setprecision(18) << ans << endl;
    return 0;
}