#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
#define TIME (clock() * 1.0 / CLOCKS_PER_SEC)

const int M = 1e5 + 239;

int n, w[M];
int tot;
ld dp[M];
int cur[M];
int sm[M];

void solve() {
    cin >> n;
    tot = 0;
    for (int i = 0; i < n; i++) {
        cin >> w[i];
        tot += w[i];
    }
    for (int x = 1; x <= n; x++) {
        for (int z = x; z <= n; z += x) {
            sm[z] += w[x - 1];
        }
    }
    for (int i = n - 1; i >= 0; i--) {
        int x = i + 1;
        for (int z = x; z <= n; z += x) {
            cur[z / x] = sm[z];
        }
        for (int u = 1; u <= (n / x); u++) {
            for (int v = 2 * u; v <= (n / x); v += u) {
                cur[v] -= cur[u];
            }
        }
        int rhs = 0;
        for (int u = 1; u <= (n / x); u++) {
            rhs += cur[u];
        }
        ld res = rhs;
        for (int u = 2; u <= (n / x); u++) {
            res += dp[x * u - 1] * cur[u];
        }
        dp[i] = res / (tot - cur[1]);
    }
    cout << dp[0] << "\n";
}

int main() {
#ifdef ONPC
    freopen("input", "r", stdin);
#endif
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);

    cout << fixed << setprecision(20);

    int t = 1;
    //cin >> t;
    while (t--) {
        solve();
    }

    return 0;
}