#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;

constexpr int N = 1e5 + 500;

int main() {
    cin.tie(0)->sync_with_stdio(false);
    vector<char> sv(N + 1, 0);
    vector<vector<ll>> divs(N + 1);
    vector<vector<ld>> pref(N + 1);
    vector<map<int, int>> primes(N + 1);
    sv[0] = sv[1] = 1;
    for (ll i = 2; i <= N; ++i) {
        if (sv[i])continue;
        for (ll j = i; j <= N; j += i) {
            sv[j] = 1;
            ll cur = i;
            while (j % cur == 0) {
                primes[j][i] = cur;
                cur *= i;
            }
        }
    }
    for (ll i = 1; i <= N; ++i) {
        ll t = 0;
        for (ll j = i; j <= N; j += i) {
            divs[j].push_back(i);
            ++t;
        }
    }
    int n = 1e5;
    cin >> n;
    vector<ll> w(n);
    ll s = 0;
    for (auto &i: w) {
        i = 1;
        cin >> i;
        s += i;
    }
    vector<ld> prob(n + 1, 0);
    for (int i = 1; i <= n; ++i) {
        prob[i] = (ld) w[i - 1] / s;
    }
    vector<ld> dp(n + 1, 0);
    for (int i = n; i; --i) {
        ld o = 0;
        ld p = 0.0;
        for (auto d: divs[i])p += prob[d];
        ld total = p;
        for (int j = i * 2; j <= n; j += i) {
            ll val = 1;
            ll x = i;
            for (auto [a, b]: primes[j]) {
                if (!primes[i].count(a)) {
                    val *= b;
                } else if (primes[i][a] < b) {
                    val *= b;
                    x /= primes[i][a];
                }
            }
            ld z = 0.0;
            for (auto d: divs[x]) {
                if (d * val > n)break;
                z += prob[d * val];
            }
            total += z;
            o += dp[j] * z;
        }
        dp[i] = (total + o) / (1 - p);
    }
    cout << fixed << setprecision(20);
    cout << dp[1];
}