#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    using ld = long double;
    ll N,M; cin >> N >> M;
    vector<ll> B(M);
    rep(i,M) cin >> B[i];
    vector<ll> B_sum(M + 1, 0);
    rep(i,M) B_sum[i + 1] += B_sum[i] + B[i];

    ld ans = 1e18;
    rep(i,M) {
        ld cur = 0;
        cur += M / ld(M - i);
        cur += ((M - i) * N - (B_sum[M] - B_sum[i])) / ld(M - i);
        ans = min(ans, cur);
    }
    ans += B[0] - 1.0;
    ans = min(ans, ld(N - 1));
    cout << fixed << setprecision(20) << ans << endl;
}